[X86MCInstLower] Clean up EmitNops; NFC

Instead of having a conditional assert inside EmitNops, refactor so that
the caller can have the assert instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266793 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed