[AArch64, PATCH] Improve Neon store of zero

[Jackson Woodruff <jackson dot woodruff at foss dot arm dot com>]

Hi all,

This patch changes patterns in aarch64-simd.md to replace

    movi    v0.4s, 0
    str    q0, [x0, 16]

With:

    stp xzr, xzr, [x0, 16]

When we are storing zeros to vectors like this:

    void f(uint32x4_t *p) {
      uint32x4_t x = { 0, 0, 0, 0};
      p[1] = x;
    }

Bootstrapped and regtested on aarch64 with no regressions.
OK for trunk?

Jackson

gcc/

2017-08-09  Jackson Woodruff  <jackson.woodruff@arm.com>

	* aarch64-simd.md (mov<mode>): No longer force zero
	immediate into register.
	(*aarch64_simd_mov<mode>): Add new case for stp
	using zero immediate.


gcc/testsuite

2017-08-09  Jackson Woodruff  <jackson.woodruff@arm.com>

	* gcc.target/aarch64/simd/neon_str_zero.c: New.

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