[llvm] [AArch64][GlobalISel] Support more types for TRUNC (PR #66927)

[Amara Emerson via llvm-commits]

================
@@ -6019,6 +6015,57 @@ LegalizerHelper::LegalizeResult LegalizerHelper::lowerEXT(MachineInstr &MI) {
   return UnableToLegalize;
 }
 
+LegalizerHelper::LegalizeResult LegalizerHelper::lowerTRUNC(MachineInstr &MI) {
+  // MachineIRBuilder &MIRBuilder = Helper.MIRBuilder;
+  MachineRegisterInfo &MRI = *MIRBuilder.getMRI();
+  // Similar to how operand splitting is done in SelectiondDAG, we can handle
+  // %res(v8s8) = G_TRUNC %in(v8s32) by generating:
+  //   %inlo(<4x s32>), %inhi(<4 x s32>) = G_UNMERGE %in(<8 x s32>)
+  //   %lo16(<4 x s16>) = G_TRUNC %inlo
+  //   %hi16(<4 x s16>) = G_TRUNC %inhi
+  //   %in16(<8 x s16>) = G_CONCAT_VECTORS %lo16, %hi16
+  //   %res(<8 x s8>) = G_TRUNC %in16
+
+  Register DstReg = MI.getOperand(0).getReg();
+  Register SrcReg = MI.getOperand(1).getReg();
+  LLT DstTy = MRI.getType(DstReg);
+  LLT SrcTy = MRI.getType(SrcReg);
+
+  assert(DstTy.isVector() && "This should be a vector operation");
----------------
aemerson wrote:

lower() shouldn't die on the wrong types. It should return `UnableToLegalize`.

And do we also need to check for even num elts?

https://github.com/llvm/llvm-project/pull/66927