[x86] Bypass the shuffle mask comment generation when not using verbose

[x86] Bypass the shuffle mask comment generation when not using verbose
asm. This can be somewhat expensive and there is no reason to do it
outside of tests or debugging sessions. I'm also likely to make it
significantly more expensive to support more styles of shuffles.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218362 91177308-0d34-0410-b5e6-96231b3b80d8