PR tree-optimization/101895: Fold VEC_PERM to help recognize FMA.

PR tree-optimization/101895: Fold VEC_PERM to help recognize FMA.

This patch resolves PR tree-optimization/101895 a missed optimization
regression, by adding a costant folding simplification to match.pd to
simplify the transform "mult; vec_perm; plus" into "vec_perm; mult; plus"
with the aim that keeping the multiplication and addition next to each
other allows them to be recognized as fused-multiply-add on suitable
targets.  This transformation requires a tweak to match.pd's
vec_same_elem_p predicate to handle CONSTRUCTOR_EXPRs using the same
SSA_NAME_DEF_STMT idiom used for constructors elsewhere in match.pd.

The net effect is that the following code example:

void foo(float * __restrict__ a, float b, float *c) {
  a[0] = c[0]*b + a[0];
  a[1] = c[2]*b + a[1];
  a[2] = c[1]*b + a[2];
  a[3] = c[3]*b + a[3];
}

when compiled on x86_64-pc-linux-gnu with -O2 -march=cascadelake
currently generates:

        vbroadcastss    %xmm0, %xmm0
        vmulps  (%rsi), %xmm0, %xmm0
        vpermilps       $216, %xmm0, %xmm0
        vaddps  (%rdi), %xmm0, %xmm0
        vmovups %xmm0, (%rdi)
        ret

but with this patch now generates the improved:

        vpermilps       $216, (%rsi), %xmm1
        vbroadcastss    %xmm0, %xmm0
        vfmadd213ps     (%rdi), %xmm0, %xmm1
        vmovups %xmm1, (%rdi)
        ret

2022-03-15  Roger Sayle  <roger@nextmovesoftware.com>
	    Marc Glisse  <marc.glisse@inria.fr>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	PR tree-optimization/101895
	* match.pd (vec_same_elem_p): Handle CONSTRUCTOR_EXPR def.
	(plus (vec_perm (mult ...) ...) ...): New reordering simplification.

gcc/testsuite/ChangeLog
	PR tree-optimization/101895
	* gcc.target/i386/pr101895.c: New test case.