[llvm] [AMDGPU] Accept extractelement of a widening cast when folding image ops to a16 (PR #208207)

[Sebastian Neubauer via llvm-commits]

================
@@ -115,6 +125,14 @@ static Value *convertTo16Bit(Value &V, InstCombiner::BuilderTy &Builder) {
   Type *VTy = V.getType();
   if (isa<FPExtInst, SExtInst, ZExtInst>(&V))
     return cast<Instruction>(&V)->getOperand(0);
+  // Vector form: extractelement((s|z|fp)ext Vec), Idx -> extractelement(Vec,
+  // Idx), taking the narrow lane directly so the widening cast can be removed.
+  Value *VecCast, *Idx;
+  if (match(&V, m_ExtractElt(PatternMatch::m_Value(VecCast),
+                             PatternMatch::m_Value(Idx))) &&
+      isa<FPExtInst, SExtInst, ZExtInst>(VecCast))
+    return Builder.CreateExtractElement(
+        cast<Instruction>(VecCast)->getOperand(0), Idx);
----------------
Flakebi wrote:

Should be able to use `m_Instruction` instead of `m_Value` above and eliminate the cast here

https://github.com/llvm/llvm-project/pull/208207