[llvm] [LLVM] treat `@llvm.ssub.sat` the same as `@llvm.aarch64.neon.sqsub` (PR #140454)

[via llvm-commits]

================
@@ -1795,6 +1795,34 @@ bool AArch64LegalizerInfo::legalizeIntrinsic(LegalizerHelper &Helper,
     MI.eraseFromParent();
     return true;
   }
+  case Intrinsic::aarch64_neon_sqadd: {
+    MachineIRBuilder MIB(MI);
+    if (MIB.getMRI()->getType(MI.getOperand(0).getReg()).isVector())
----------------
SpencerAbson wrote:

I feel like we should be able to use `MachineIRBuilder &MIB = Helper.MIRBuilder;` throughout this function (apologies for not pulling and testing).  Either way, we ought to capture it in the `LowerBinop` lambda instead of building a new one there too.

https://github.com/llvm/llvm-project/pull/140454