add one more case of mismatched input/output constraints.

When the mismatch is due to a larger input operand that is
a constant, truncate it down to the size of the output.  This
allows us to accept some cases in the linux kernel and elsewhere.
Pedantically speaking, we generate different code than GCC, though
I can't imagine how it would matter:

Clang:
	movb	$-1, %al
	frob %al

GCC:

	movl	$255, %eax
	frob %al

llvm-svn: 126148