Don't crash when we see unallocatable registers in clobbers

Don't crash when we see unallocatable registers in clobbers

This fixes a bug where we'd crash given code like the test-case from
https://bugs.llvm.org/show_bug.cgi?id=30792 . Instead, we let the
offending clobber silently slide through.

This doesn't fully fix said bug, since the assembler will still complain
the moment it sees a crypto/fp/vector op, and we still don't diagnose
calls that require vector regs.

Differential Revision: https://reviews.llvm.org/D39030

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316374 91177308-0d34-0410-b5e6-96231b3b80d8