[GlobalISel] Simplify narrowScalarMul. NFC.

Remove some redundancy because the source and result types of any
multiply are always the same.

Author: jayfoad

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -5337,26 +5337,23 @@ LegalizerHelper::narrowScalarMul(MachineInstr &MI, LLT NarrowTy) {
   if (Ty.isVector())
     return UnableToLegalize;
 
-  unsigned SrcSize = MRI.getType(Src1).getSizeInBits();
-  unsigned DstSize = Ty.getSizeInBits();
+  unsigned Size = Ty.getSizeInBits();
   unsigned NarrowSize = NarrowTy.getSizeInBits();
-  if (DstSize % NarrowSize != 0 || SrcSize % NarrowSize != 0)
+  if (Size % NarrowSize != 0)
     return UnableToLegalize;
 
-  unsigned NumDstParts = DstSize / NarrowSize;
-  unsigned NumSrcParts = SrcSize / NarrowSize;
+  unsigned NumParts = Size / NarrowSize;
   bool IsMulHigh = MI.getOpcode() == TargetOpcode::G_UMULH;
-  unsigned DstTmpParts = NumDstParts * (IsMulHigh ? 2 : 1);
+  unsigned DstTmpParts = NumParts * (IsMulHigh ? 2 : 1);
 
   SmallVector<Register, 2> Src1Parts, Src2Parts;
   SmallVector<Register, 2> DstTmpRegs(DstTmpParts);
-  extractParts(Src1, NarrowTy, NumSrcParts, Src1Parts);
-  extractParts(Src2, NarrowTy, NumSrcParts, Src2Parts);
+  extractParts(Src1, NarrowTy, NumParts, Src1Parts);
+  extractParts(Src2, NarrowTy, NumParts, Src2Parts);
   multiplyRegisters(DstTmpRegs, Src1Parts, Src2Parts, NarrowTy);
 
   // Take only high half of registers if this is high mul.
-  ArrayRef<Register> DstRegs(
-      IsMulHigh ? &DstTmpRegs[DstTmpParts / 2] : &DstTmpRegs[0], NumDstParts);
+  ArrayRef<Register> DstRegs(&DstTmpRegs[DstTmpParts - NumParts], NumParts);
   MIRBuilder.buildMerge(DstReg, DstRegs);
   MI.eraseFromParent();
   return Legalized;