[llvm] [AArch64] When lowering 16 bit vector bitcasts, convert to i16. (PR #178249)

[Cullen Rhodes via llvm-commits]

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@@ -5594,10 +5594,17 @@ SDValue AArch64TargetLowering::LowerBITCAST(SDValue Op,
   if (ArgVT == MVT::f16 || ArgVT == MVT::bf16)
     return Op;
 
+  SDValue Src = Op.getOperand(0);
+  if (ArgVT.isVector() && ArgVT.getSizeInBits() == 16) {
+    SDLoc DL(Op);
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c-rhodes wrote:

nit: I'd move this above so it can be reused below

https://github.com/llvm/llvm-project/pull/178249