[arm] Improve constant handling for usubvsi4.

[arm] Improve constant handling for usubvsi4.

This patch improves the expansion of usubvsi4 by allowing suitable
constants to be passed directly.  Unlike normal subtraction, either
operand may be a constant (and indeed I have seen cases where both can
be with LTO enabled).  One interesting testcase that improves as a
result of this is:

unsigned f6 (unsigned a)
{
  unsigned x;
  return __builtin_sub_overflow (5U, a, &x) ? 0 : x;
}

Which previously compiled to:

	rsbs	r3, r0, #5
	cmp	r0, #5
	movls	r0, r3
	movhi	r0, #0

but now generates the optimal sequence:

	rsbs	r0, r0, #5
	movcc	r0, #0

	* config/arm/arm.md (usubv<mode>4): Delete expansion.
	(usubvsi4): New pattern.  Allow some immediate values for inputs.
	(usubvdi4): New pattern.

From-SVN: r277187