[llvm] [AMDGPU] Init pointers, widen mask complements (NFC) (PR #208469)

[Matt Arsenault via llvm-commits]

================
@@ -4942,7 +4942,7 @@ bool AMDGPULegalizerInfo::loadInputValue(
     AMDGPUFunctionArgInfo::PreloadedValue ArgType) const {
   const SIMachineFunctionInfo *MFI = B.getMF().getInfo<SIMachineFunctionInfo>();
   const ArgDescriptor *Arg = nullptr;
-  const TargetRegisterClass *ArgRC;
+  const TargetRegisterClass *ArgRC = nullptr;
----------------
arsenm wrote:

The warning here is probably wrong 

https://github.com/llvm/llvm-project/pull/208469