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

llvm-svn: 316374