[AArch64][GlobalISel] Use the look-through constant helper for the shift s32->s64 custom legalization.

Almost NFC, except it catches more cases and gives a 0.1% CTMark -O0 size win.

Author: aemerson

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp

@@ -710,16 +710,14 @@ bool AArch64LegalizerInfo::legalizeShlAshrLshr(
   // If the shift amount is a G_CONSTANT, promote it to a 64 bit type so the
   // imported patterns can select it later. Either way, it will be legal.
   Register AmtReg = MI.getOperand(2).getReg();
-  auto *CstMI = MRI.getVRegDef(AmtReg);
-  assert(CstMI && "expected to find a vreg def");
-  if (CstMI->getOpcode() != TargetOpcode::G_CONSTANT)
+  auto VRegAndVal = getConstantVRegValWithLookThrough(AmtReg, MRI);
+  if (!VRegAndVal)
     return true;
   // Check the shift amount is in range for an immediate form.
-  unsigned Amount = CstMI->getOperand(1).getCImm()->getZExtValue();
+  int64_t Amount = VRegAndVal->Value;
   if (Amount > 31)
     return true; // This will have to remain a register variant.
-  assert(MRI.getType(AmtReg).getSizeInBits() == 32);
-  auto ExtCst = MIRBuilder.buildZExt(LLT::scalar(64), AmtReg);
+  auto ExtCst = MIRBuilder.buildConstant(LLT::scalar(64), Amount);
   MI.getOperand(2).setReg(ExtCst.getReg(0));
   return true;
 }

llvm/test/CodeGen/AArch64/GlobalISel/legalize-unmerge-values.mir

@@ -24,9 +24,10 @@ body: |
     ; CHECK-LABEL: name: test_unmerge_s4
     ; CHECK: [[COPY:%[0-9]+]]:_(s32) = COPY $w0
     ; CHECK: [[UV:%[0-9]+]]:_(s8), [[UV1:%[0-9]+]]:_(s8), [[UV2:%[0-9]+]]:_(s8), [[UV3:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[COPY]](s32)
-    ; CHECK: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 4
+    ; CHECK: [[C:%[0-9]+]]:_(s8) = G_CONSTANT i8 4
     ; CHECK: [[ZEXT:%[0-9]+]]:_(s32) = G_ZEXT [[UV]](s8)
-    ; CHECK: [[LSHR:%[0-9]+]]:_(s32) = G_LSHR [[ZEXT]], [[C]](s32)
+    ; CHECK: [[C1:%[0-9]+]]:_(s64) = G_CONSTANT i64 4
+    ; CHECK: [[LSHR:%[0-9]+]]:_(s32) = G_LSHR [[ZEXT]], [[C1]](s64)
     ; CHECK: [[ANYEXT:%[0-9]+]]:_(s64) = G_ANYEXT [[UV]](s8)
     ; CHECK: [[ANYEXT1:%[0-9]+]]:_(s64) = G_ANYEXT [[LSHR]](s32)
     ; CHECK: $x0 = COPY [[ANYEXT]](s64)