AMDGPU: Fix -Wextra (llvm#138539)

Another stupid gcc warning. Ideally we would directly use the enum type,
but subregister indexes are emitted as an anonymous enum.

Fixes llvm#125548

Author: arsenm

llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp

@@ -2521,8 +2521,9 @@ bool AMDGPUInstructionSelector::selectG_TRUNC(MachineInstr &I) const {
     return false;
 
   if (SrcSize > 32) {
-    unsigned SubRegIdx =
-        DstSize < 32 ? AMDGPU::sub0 : TRI.getSubRegFromChannel(0, DstSize / 32);
+    unsigned SubRegIdx = DstSize < 32
+                             ? static_cast<unsigned>(AMDGPU::sub0)
+                             : TRI.getSubRegFromChannel(0, DstSize / 32);
     if (SubRegIdx == AMDGPU::NoSubRegister)
       return false;