[AArch64] Refactor instructions that write PSTATE (NFCI)

[olista01]

Summary

Reuse some code in preparation for the v8.5A XAFlag/AXFlag instructions,
which shares part of the encoding of the MSR-immediate.

Patch by Pablo Barrio!

lib/Target/AArch64/AArch64InstrFormats.td

def SystemPStateFieldWithImm0_15Operand : AsmOperandClass {
   let Name = "SystemPStateFieldWithImm0_15";
   let ParserMethod = "tryParseSysReg";
 }
 def pstatefield4_op : Operand<i32> {
   let ParserMatchClass = SystemPStateFieldWithImm0_15Operand;
   let PrintMethod = "printSystemPStateField";
 }
 
+// Instructions to modify PSTATE, no input reg
 let Defs = [NZCV] in
+class PstateWriteSimple<dag iops, string asm, string operands>
+  : SimpleSystemI<0, iops, asm, operands> {
+
+  let Inst{20-19} = 0b00;
+  let Inst{15-12} = 0b0100;
+}
+
 class MSRpstateImm0_15
-  : SimpleSystemI<0, (ins pstatefield4_op:$pstatefield, imm0_15:$imm),
-                  "msr", "\t$pstatefield, $imm">,
+  : PstateWriteSimple<(ins pstatefield4_op:$pstatefield, imm0_15:$imm), "msr",
+                  "\t$pstatefield, $imm">,
     Sched<[WriteSys]> {
+
   bits<6> pstatefield;
   bits<4> imm;
-  let Inst{20-19} = 0b00;
   let Inst{18-16} = pstatefield{5-3};
-  let Inst{15-12} = 0b0100;
   let Inst{11-8} = imm;
   let Inst{7-5} = pstatefield{2-0};
 
   let DecoderMethod = "DecodeSystemPStateInstruction";
   // MSRpstateI aliases with MSRI. When the MSRpstateI decoder method returns
   // Fail the decoder should attempt to decode the instruction as MSRI.
   let hasCompleteDecoder = 0;
 }
 
 def SystemPStateFieldWithImm0_1Operand : AsmOperandClass {
   let Name = "SystemPStateFieldWithImm0_1";
   let ParserMethod = "tryParseSysReg";
 }
 def pstatefield1_op : Operand<i32> {
   let ParserMatchClass = SystemPStateFieldWithImm0_1Operand;
   let PrintMethod = "printSystemPStateField";
 }
 
-let Defs = [NZCV] in
 class MSRpstateImm0_1
-  : SimpleSystemI<0, (ins pstatefield1_op:$pstatefield, imm0_1:$imm),
-                  "msr", "\t$pstatefield, $imm">,
+  : PstateWriteSimple<(ins pstatefield1_op:$pstatefield, imm0_1:$imm), "msr",
+                 "\t$pstatefield, $imm">,
     Sched<[WriteSys]> {
+
   bits<6> pstatefield;
   bit imm;
-  let Inst{20-19} = 0b00;
   let Inst{18-16} = pstatefield{5-3};
-  let Inst{15-9} = 0b0100000;
+  let Inst{11-9} = 0b000;
   let Inst{8} = imm;
   let Inst{7-5} = pstatefield{2-0};
 
   let DecoderMethod = "DecodeSystemPStateInstruction";
   // MSRpstateI aliases with MSRI. When the MSRpstateI decoder method returns
   // Fail the decoder should attempt to decode the instruction as MSRI.
   let hasCompleteDecoder = 0;
 }