[MachineCombiner] Don't ignore PHI depths (llvm#82025)

The depths of the Root and the NewRoot are to be compared in
MachineCombiner::improvesCriticalPathLen(), and while the call to
BlockTrace.getInstrCycles(*Root) includes the Depth of a PHI, for some
reason PHI nodes have been ignored in getOperandDef(). 

This patch removes the special handling of PHIs in getOperandDef() so that
Root and NewRoot get a fair comparison. This does not affect loop headers
as MachineTraceMetrics handles that case by ignoring incoming PHI edges.

Author: JonPsson1

llvm/lib/CodeGen/MachineCombiner.cpp

@@ -155,9 +155,6 @@ MachineCombiner::getOperandDef(const MachineOperand &MO) {
   // We need a virtual register definition.
   if (MO.isReg() && MO.getReg().isVirtual())
     DefInstr = MRI->getUniqueVRegDef(MO.getReg());
-  // PHI's have no depth etc.
-  if (DefInstr && DefInstr->isPHI())
-    DefInstr = nullptr;
   return DefInstr;
 }
 

llvm/test/CodeGen/RISCV/umulo-128-legalisation-lowering.ll

@@ -89,8 +89,8 @@ define { i128, i8 } @muloti_test(i128 %l, i128 %r) #0 {
 ; RISCV32-NEXT:    snez a3, a3
 ; RISCV32-NEXT:    and a3, a3, a7
 ; RISCV32-NEXT:    or a2, a3, a2
-; RISCV32-NEXT:    or a3, t2, t3
-; RISCV32-NEXT:    or a2, a2, a3
+; RISCV32-NEXT:    or a2, a2, t2
+; RISCV32-NEXT:    or a2, a2, t3
 ; RISCV32-NEXT:    mul a3, a5, a4
 ; RISCV32-NEXT:    andi a2, a2, 1
 ; RISCV32-NEXT:    sw a3, 0(a0)

llvm/test/CodeGen/X86/bitcast-and-setcc-256.ll

@@ -553,8 +553,8 @@ define i8 @v8i32_or_select(<8 x i32> %a0, <8 x i32> %a1, <8 x i32> %a2, <8 x i32
 ; AVX1-NEXT:    vpcmpeqd %xmm5, %xmm4, %xmm4
 ; AVX1-NEXT:    vpcmpeqd %xmm1, %xmm0, %xmm0
 ; AVX1-NEXT:    vinsertf128 $1, %xmm4, %ymm0, %ymm0
-; AVX1-NEXT:    vorps %ymm2, %ymm3, %ymm1
-; AVX1-NEXT:    vorps %ymm0, %ymm1, %ymm0
+; AVX1-NEXT:    vorps %ymm0, %ymm3, %ymm0
+; AVX1-NEXT:    vorps %ymm2, %ymm0, %ymm0
 ; AVX1-NEXT:    vmovmskps %ymm0, %eax
 ; AVX1-NEXT:    # kill: def $al killed $al killed $eax
 ; AVX1-NEXT:    vzeroupper
@@ -571,8 +571,8 @@ define i8 @v8i32_or_select(<8 x i32> %a0, <8 x i32> %a1, <8 x i32> %a2, <8 x i32
 ; AVX2-NEXT:    vpcmpeqd %ymm2, %ymm0, %ymm2
 ; AVX2-NEXT:  .LBB7_3:
 ; AVX2-NEXT:    vpcmpeqd %ymm1, %ymm0, %ymm0
-; AVX2-NEXT:    vpor %ymm2, %ymm3, %ymm1
-; AVX2-NEXT:    vpor %ymm0, %ymm1, %ymm0
+; AVX2-NEXT:    vpor %ymm0, %ymm3, %ymm0
+; AVX2-NEXT:    vpor %ymm2, %ymm0, %ymm0
 ; AVX2-NEXT:    vmovmskps %ymm0, %eax
 ; AVX2-NEXT:    # kill: def $al killed $al killed $eax
 ; AVX2-NEXT:    vzeroupper