[AMDGPU] More accurate names for dpp operand types

[Joe_Nash]

Summary

NFC. Renames the variable in the dpp input operand generators
from DstRC to OldRC, because that is what it actually sets.

Also documents the importance of setting HasModifiers = 0 in the
dpp8 asm string.

llvm/lib/Target/AMDGPU/SIInstrInfo.td

class getInsVOP3OpSel <RegisterOperand Src0RC, RegisterOperand Src1RC,
                        bit HasClamp, bit HasOMod,
                        Operand Src0Mod, Operand Src1Mod, Operand Src2Mod> {
   dag ret = getInsVOP3Base<Src0RC, Src1RC,
                     Src2RC, NumSrcArgs,
                     HasClamp, 1/*HasModifiers*/, 1/*HasSrc2Mods*/, HasOMod,
                     Src0Mod, Src1Mod, Src2Mod, 1/*HasOpSel*/, 0>.ret;
 }
 
-class getInsDPPBase <RegisterOperand DstRC, RegisterClass Src0RC, RegisterClass Src1RC,
+class getInsDPPBase <RegisterOperand OldRC, RegisterClass Src0RC, RegisterClass Src1RC,
                  int NumSrcArgs, bit HasModifiers,
                  Operand Src0Mod, Operand Src1Mod> {
 
   dag ret = !if (!eq(NumSrcArgs, 0),
                 // VOP1 without input operands (V_NOP)
                 (ins ),
             !if (!eq(NumSrcArgs, 1),
               !if (HasModifiers,
                 // VOP1_DPP with modifiers
-                (ins DstRC:$old, Src0Mod:$src0_modifiers,
+                (ins OldRC:$old, Src0Mod:$src0_modifiers,
                      Src0RC:$src0)
               /* else */,
                 // VOP1_DPP without modifiers
-                (ins DstRC:$old, Src0RC:$src0)
+                (ins OldRC:$old, Src0RC:$src0)
               /* endif */),
               !if (HasModifiers,
                 // VOP2_DPP with modifiers
-                (ins DstRC:$old,
+                (ins OldRC:$old,
                      Src0Mod:$src0_modifiers, Src0RC:$src0,
                      Src1Mod:$src1_modifiers, Src1RC:$src1)
               /* else */,
                 // VOP2_DPP without modifiers
-                (ins DstRC:$old,
+                (ins OldRC:$old,
                      Src0RC:$src0, Src1RC:$src1)
                 )));
 }
 
-class getInsDPP <RegisterOperand DstRC, RegisterClass Src0RC, RegisterClass Src1RC,
+class getInsDPP <RegisterOperand OldRC, RegisterClass Src0RC, RegisterClass Src1RC,
                  int NumSrcArgs, bit HasModifiers,
                  Operand Src0Mod, Operand Src1Mod> {
-  dag ret = !con(getInsDPPBase<DstRC, Src0RC, Src1RC, NumSrcArgs,
+  dag ret = !con(getInsDPPBase<OldRC, Src0RC, Src1RC, NumSrcArgs,
                                HasModifiers, Src0Mod, Src1Mod>.ret,
                  (ins dpp_ctrl:$dpp_ctrl, row_mask:$row_mask,
                   bank_mask:$bank_mask, bound_ctrl:$bound_ctrl));
 }
 
-class getInsDPP16 <RegisterOperand DstRC, RegisterClass Src0RC, RegisterClass Src1RC,
+class getInsDPP16 <RegisterOperand OldRC, RegisterClass Src0RC, RegisterClass Src1RC,
                  int NumSrcArgs, bit HasModifiers,
                  Operand Src0Mod, Operand Src1Mod> {
-  dag ret = !con(getInsDPP<DstRC, Src0RC, Src1RC, NumSrcArgs,
+  dag ret = !con(getInsDPP<OldRC, Src0RC, Src1RC, NumSrcArgs,
                            HasModifiers, Src0Mod, Src1Mod>.ret,
                  (ins FI:$fi));
 }
 
-class getInsDPP8 <RegisterOperand DstRC, RegisterClass Src0RC, RegisterClass Src1RC,
+class getInsDPP8 <RegisterOperand OldRC, RegisterClass Src0RC, RegisterClass Src1RC,
                  int NumSrcArgs, bit HasModifiers,
                  Operand Src0Mod, Operand Src1Mod> {
-  dag ret = !con(getInsDPPBase<DstRC, Src0RC, Src1RC, NumSrcArgs,
+  dag ret = !con(getInsDPPBase<OldRC, Src0RC, Src1RC, NumSrcArgs,
                                HasModifiers, Src0Mod, Src1Mod>.ret,
                  (ins dpp8:$dpp8, FI:$fi));
 }
 
 
 // Ins for SDWA
 class getInsSDWA <RegisterOperand Src0RC, RegisterOperand Src1RC, int NumSrcArgs,
                   bit HasSDWAOMod, Operand Src0Mod, Operand Src1Mod,
class VOPProfile <list<ValueType> _ArgVT, bit _EnableF32SrcMods = 0,
   field string AsmVOP3OpSel = getAsmVOP3OpSel<NumSrcArgs,
                                               HasClamp, HasOMod,
                                               HasSrc0FloatMods,
                                               HasSrc1FloatMods,
                                               HasSrc2FloatMods>.ret;
   field string AsmDPP = !if(HasExtDPP,
                             getAsmDPP<HasDst, NumSrcArgs, HasModifiers, DstVT>.ret, "");
   field string AsmDPP16 = getAsmDPP16<HasDst, NumSrcArgs, HasModifiers, DstVT>.ret;
-  field string AsmDPP8 = getAsmDPP8<HasDst, NumSrcArgs, 0, DstVT>.ret;
+  // DPP8 encoding has no fields for modifiers, and it is enforced by setting
+  // the asm operand name via this HasModifiers flag
+  field string AsmDPP8 = getAsmDPP8<HasDst, NumSrcArgs, 0 /*HasModifiers*/, DstVT>.ret;
   field string AsmSDWA = getAsmSDWA<HasDst, NumSrcArgs, DstVT>.ret;
   field string AsmSDWA9 = getAsmSDWA9<HasDst, HasSDWAOMod, NumSrcArgs, DstVT>.ret;
 
   field string TieRegDPP = "$old";
 }
 
 class VOP_NO_EXT <VOPProfile p> : VOPProfile <p.ArgVT> {
   let HasExt = 0;