As a follow-up to r251566, do the same for the other optionally-supported
register classes (mostly for vector registers). Don't return an unavailable
register class (which would cause an assert later), but fail cleanly when
provided an unsupported inline asm constraint.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251575 91177308-0d34-0410-b5e6-96231b3b80d8
Hal Finkel