[NFC][AMDGPU] Guard the custom fixups kind array from invalid access

Protect from accidental passing of an invalid MCFixupKind value which
can cause an out-of-bounds access in the array.

Reviewed by: arsenm

Differential Revision: https://reviews.llvm.org/D158725

Author: GeorgeWeb

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp

@@ -185,6 +185,8 @@ const MCFixupKindInfo &AMDGPUAsmBackend::getFixupKindInfo(
   if (Kind < FirstTargetFixupKind)
     return MCAsmBackend::getFixupKindInfo(Kind);
 
+  assert(unsigned(Kind - FirstTargetFixupKind) < getNumFixupKinds() &&
+         "Invalid kind!");
   return Infos[Kind - FirstTargetFixupKind];
 }