[AMDGPU] Allow multiple uses of the same literal in SOP2/SOPC

[dstuttard]

LGTM

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

if ((getNamedOperand(MI, AMDGPU::OpName::src0_modifiers)->getImm() &
       ErrInfo = "ABS not allowed in VOP3B instructions";
       return false;
     }
   }
 
   if (isSOP2(MI) || isSOPC(MI)) {
     const MachineOperand &Src0 = MI.getOperand(Src0Idx);
     const MachineOperand &Src1 = MI.getOperand(Src1Idx);
-    unsigned Immediates = 0;
 
-    if (!Src0.isReg() &&
-        !isInlineConstant(Src0, Desc.OpInfo[Src0Idx].OperandType))
-      Immediates++;
-    if (!Src1.isReg() &&
-        !isInlineConstant(Src1, Desc.OpInfo[Src1Idx].OperandType))
-      Immediates++;
-
-    if (Immediates > 1) {
+    if (!Src0.isReg() && !Src1.isReg() &&
+        !isInlineConstant(Src0, Desc.OpInfo[Src0Idx].OperandType) &&
+        !isInlineConstant(Src1, Desc.OpInfo[Src1Idx].OperandType) &&
+        !Src0.isIdenticalTo(Src1)) {
       ErrInfo = "SOP2/SOPC instruction requires too many immediate constants";
       return false;
     }
   }
 
   if (isSOPK(MI)) {
     auto Op = getNamedOperand(MI, AMDGPU::OpName::simm16);
     if (Desc.isBranch()) {

llvm/test/CodeGen/AMDGPU/verify-duplicate-literal.mir

 body:            |
   bb.0:
     liveins: $vgpr0
     ; CHECK-LABEL: name: use_duplicate_literal_fma
     ; CHECK: liveins: $vgpr0
     ; CHECK: $vgpr0 = V_FMA_F32_e64 0, $vgpr0, 0, 1077936128, 0, 1077936128, 0, 0, implicit $mode, implicit $exec
     $vgpr0 = V_FMA_F32_e64 0, $vgpr0, 0, 1077936128, 0, 1077936128, 0, 0, implicit $mode, implicit $exec
 ...
+
+---
+name: use_duplicate_literal_sop2
+tracksRegLiveness: true
+body: |
+  bb.0:
+    $sgpr0 = S_ADD_U32 12345, 12345, implicit-def $scc
+...
+
+---
+name: use_duplicate_literal_sopc
+tracksRegLiveness: true
+body: |
+  bb.0:
+    S_CMP_LG_U32 305419896, 305419896, implicit-def $scc
+...