PeepholeOpt: Remove subreg def check for bitcast (#130086)

Subregister defs are illegal in SSA. Surprisingly this enables folding
into subregister insert patterns in one test.

Author: arsenm

llvm/lib/CodeGen/PeepholeOptimizer.cpp

@@ -1923,11 +1923,8 @@ ValueTrackerResult ValueTracker::getNextSourceFromBitcast() {
   // Bitcasts with more than one def are not supported.
   if (Def->getDesc().getNumDefs() != 1)
     return ValueTrackerResult();
-  const MachineOperand DefOp = Def->getOperand(DefIdx);
-  if (DefOp.getSubReg() != DefSubReg)
-    // If we look for a different subreg, it means we want a subreg of the src.
-    // Bails as we do not support composing subregs yet.
-    return ValueTrackerResult();
+
+  assert(!Def->getOperand(DefIdx).getSubReg() && "no subregister defs in SSA");
 
   unsigned SrcIdx = Def->getNumOperands();
   for (unsigned OpIdx = DefIdx + 1, EndOpIdx = SrcIdx; OpIdx != EndOpIdx;
@@ -1950,6 +1947,8 @@ ValueTrackerResult ValueTracker::getNextSourceFromBitcast() {
   if (SrcIdx >= Def->getNumOperands())
     return ValueTrackerResult();
 
+  const MachineOperand &DefOp = Def->getOperand(DefIdx);
+
   // Stop when any user of the bitcast is a SUBREG_TO_REG, replacing with a COPY
   // will break the assumed guarantees for the upper bits.
   for (const MachineInstr &UseMI : MRI.use_nodbg_instructions(DefOp.getReg())) {

llvm/test/CodeGen/X86/pr41619.ll

@@ -6,8 +6,6 @@ define void @foo(double %arg) {
 ; CHECK-LABEL: foo:
 ; CHECK:       ## %bb.0: ## %bb
 ; CHECK-NEXT:    vmovq %xmm0, %rax
-; CHECK-NEXT:    vmovd %eax, %xmm0
-; CHECK-NEXT:    vmovq %xmm0, %rax
 ; CHECK-NEXT:    movl %eax, (%rax)
 ; CHECK-NEXT:    movq $0, (%rax)
 ; CHECK-NEXT:    retq