[llvm] [GlobalIsel] Modernize truncate of ext. (PR #100338)

[Matt Arsenault via llvm-commits]

================
@@ -113,3 +113,45 @@ bool CombinerHelper::matchNonNegZext(const MachineOperand &MO,
 
   return false;
 }
+
+bool CombinerHelper::matchTruncateOfExt(const MachineInstr &Root,
+                                        const MachineInstr &ExtMI,
+                                        BuildFnTy &MatchInfo) {
+  const GTrunc *Trunc = cast<GTrunc>(&Root);
+  const GExtOp *Ext = cast<GExtOp>(&ExtMI);
+
+  if (!MRI.hasOneNonDBGUse(Ext->getReg(0)))
+    return false;
+
+  Register Dst = Trunc->getReg(0);
+  Register Src = Ext->getSrcReg();
+  LLT DstTy = MRI.getType(Dst);
+  LLT SrcTy = MRI.getType(Src);
+
+  if (SrcTy == DstTy) {
+    // The source and the destination are equally sized. We need to copy.
+    MatchInfo = [=](MachineIRBuilder &B) { B.buildCopy(Dst, Src); };
+
+    return true;
+  } else if (SrcTy.getScalarSizeInBits() < DstTy.getScalarSizeInBits()) {
+    // Protect against scalable vectors.
----------------
arsenm wrote:

That makes it make less sense? getScalarSizeInBits should still work for scalable 

https://github.com/llvm/llvm-project/pull/100338