[PATCH][Aarch64] Relational compare zero not merged into subtract

[Michael Collison]

This patch improves code generation for relational compares against zero that are not merged into a subtract instruction. This patch improves the >= and < cases.

An example of the '<' case:

int lt (int x, int y)
{
  if ((x - y) < 0)
    return 10;

  return 0;
}

Trunk generates:

lt:
	sub	w1, w0, w1
	mov	w0, 10
	cmp	w1, 0
	csel	w0, w0, wzr, lt
	ret

With the patch we can eliminate the redundant subtract and now generate:

lt:
	cmp	w0, w1
	mov	w0, 10
	csel	w0, w0, wzr, mi
	ret

Bootstrapped and tested on aarch64-linux-gnu. Okay for trunk?

2017-06-01  Michael Collison  <michael.collison@arm.com>

	* config/aarch64/aarch64-simd.md(aarch64_sub<mode>_compare0):
	New pattern.
	* testsuite/gcc.target/aarch64/cmp-2.c: New testcase.
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