X86 asm parser: Use a loop to disambiguate suffixes instead of copy paste

This works towards making the Intel syntax asm matcher use a completely
different disambiguation strategy.

No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214352 91177308-0d34-0410-b5e6-96231b3b80d8