[X86] Use 'llvm_unreachable' instead of nullptr in unreachable code to

avoid static check fail

RegClassOrBank is an object of RegClassOrRegBank, which is defined as
using llvm::RegClassOrRegBank = typedef PointerUnion<const
TargetRegisterClass *, const RegisterBank *>
so control flow can not get here. Use ""llvm_unreachable" here to avoid
"null pointer" confusion.

Patch by Shengchen Kan (skan)

Differential Revision: https://reviews.llvm.org/D62006



Signed-off-by: pengfei <pengfei.wang@intel.com>
llvm-svn: 361912