middle-end: convert negate + right shift into compare greater.

middle-end: convert negate + right shift into compare greater.

This turns an inversion of the sign bit + arithmetic right shift into a
comparison with 0.

i.e.

void fun1(int32_t *x, int n)
{
    for (int i = 0; i < (n & -16); i++)
      x[i] = (-x[i]) >> 31;
}

now generates:

.L3:
        ldr     q0, [x0]
        cmgt    v0.4s, v0.4s, #0
        str     q0, [x0], 16
        cmp     x0, x1
        bne     .L3

instead of:

.L3:
        ldr     q0, [x0]
        neg     v0.4s, v0.4s
        sshr    v0.4s, v0.4s, 31
        str     q0, [x0], 16
        cmp     x0, x1
        bne     .L3

gcc/ChangeLog:

	* match.pd: New negate+shift pattern.

gcc/testsuite/ChangeLog:

	* gcc.dg/signbit-2.c: New test.
	* gcc.dg/signbit-3.c: New test.
	* gcc.dg/signbit-4.c: New test.
	* gcc.dg/signbit-5.c: New test.
	* gcc.dg/signbit-6.c: New test.
	* gcc.target/aarch64/signbit-1.c: New test.