AMDGPU: Don't handle invalid reg classes in helper functions

No tests hit these and it would be better to have checks like
this explicit where they are used.

llvm-svn: 248655

llvm/lib/Target/AMDGPU/SIRegisterInfo.h

@@ -48,17 +48,11 @@ public:
 
   /// \returns true if this class contains only SGPR registers
   bool isSGPRClass(const TargetRegisterClass *RC) const {
-    if (!RC)
-      return false;
-
     return !hasVGPRs(RC);
   }
 
   /// \returns true if this class ID contains only SGPR registers
   bool isSGPRClassID(unsigned RCID) const {
-    if (static_cast<int>(RCID) == -1)
-      return false;
-
     return isSGPRClass(getRegClass(RCID));
   }