[AArch64, Patch] Generate MLA when multiply + add vector by scalar

[AArch64, Patch] Generate MLA when multiply + add vector by scalar

(On behalf of jackson.woodruff@arm.com)

This merges vector multiplies and adds into a single mla instruction
when the multiplication is done by a scalar.

    typedef int __attribute__((vector_size(16))) vec;

    vec
    mla1(vec v0, vec v1, int v2)
    {
      return v0 + v1 * c;
    }

Now generates:

    mla1:
        fmov   s2, w0
        mla    v0.4s, v1.4s, v2.s[0]

This is also done for the identical case for a multiply followed by a
subtract of vectors with an integer operand on the multiply.

gcc/

2017-07-24  Jackson Woodruff  <jackson.woodruff@arm.com>

        * config/aarch64/aarch64-simd.md (aarch64_mla_elt_merge<mode>): New.
        (aarch64_mls_elt_merge<mode>): Likewise.

gcc/testsuite/

2017-07-24  Jackson Woodruff  <jackson.woodruff@arm.com>

	* gcc.target/aarch64/simd/vmla_elem_1.c: New.

From-SVN: r250475