[GlobalISel][NFC] Fix LLT Propagation (#119587)

Retain LLT type information by creating new LLTs from the original LLT
instead of only using the original scalar size.

This PR prepares for the [LLT FPInfo
RFC](https://discourse.llvm.org/t/rfc-globalisel-adding-fp-type-information-to-llt/83349/24)
where LLTs will carry additional floating point type information in
addition to the scalar size.

Author: tgymnich

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -5347,9 +5347,9 @@ LegalizerHelper::fewerElementsBitcast(MachineInstr &MI, unsigned int TypeIdx,
 
   auto [DstReg, DstTy, SrcReg, SrcTy] = MI.getFirst2RegLLTs();
 
-  unsigned SrcScalSize = SrcTy.getScalarSizeInBits();
-  LLT SrcNarrowTy =
-      LLT::fixed_vector(NarrowTy.getSizeInBits() / SrcScalSize, SrcScalSize);
+  unsigned NewElemCount =
+      NarrowTy.getSizeInBits() / SrcTy.getScalarSizeInBits();
+  LLT SrcNarrowTy = LLT::fixed_vector(NewElemCount, SrcTy.getElementType());
 
   // Split the Src and Dst Reg into smaller registers
   SmallVector<Register> SrcVRegs, BitcastVRegs;

llvm/lib/CodeGen/GlobalISel/Utils.cpp

@@ -525,8 +525,7 @@ bool llvm::extractParts(Register Reg, LLT RegTy, LLT MainTy, LLT &LeftoverTy,
         RegNumElts % LeftoverNumElts == 0 &&
         RegTy.getScalarSizeInBits() == MainTy.getScalarSizeInBits() &&
         LeftoverNumElts > 1) {
-      LeftoverTy =
-          LLT::fixed_vector(LeftoverNumElts, RegTy.getScalarSizeInBits());
+      LeftoverTy = LLT::fixed_vector(LeftoverNumElts, RegTy.getElementType());
 
       // Unmerge the SrcReg to LeftoverTy vectors
       SmallVector<Register, 4> UnmergeValues;

llvm/lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp

@@ -154,8 +154,8 @@ static LegalizeMutation moreElementsToNextExistingRegClass(unsigned TypeIdx) {
       if (SIRegisterInfo::getSGPRClassForBitWidth(NewNumElts * EltSize))
         break;
     }
-
-    return std::pair(TypeIdx, LLT::fixed_vector(NewNumElts, EltSize));
+    return std::pair(TypeIdx,
+                     LLT::fixed_vector(NewNumElts, Ty.getElementType()));
   };
 }