tree-optimization/94864 - vector insert of vector extract simplification

The PRs ask for optimizing of

  _1 = BIT_FIELD_REF <b_3(D), 64, 64>;
  result_4 = BIT_INSERT_EXPR <a_2(D), _1, 64>;

to a vector permutation.  The following implements this as
match.pd pattern, improving code generation on x86_64.

On the RTL level we face the issue that backend patterns inconsistently
use vec_merge and vec_select of vec_concat to represent permutes.

I think using a (supported) permute is almost always better
than an extract plus insert, maybe excluding the case we extract
element zero and that's aliased to a register that can be used
directly for insertion (not sure how to query that).

The patch FAILs one case in gcc.target/i386/avx512fp16-vmovsh-1a.c
where we now expand from

 __A_28 = VEC_PERM_EXPR <x2.8_9, x1.9_10, { 0, 9, 10, 11, 12, 13, 14, 15 }>;

instead of

 _28 = BIT_FIELD_REF <x2.8_9, 16, 0>;
 __A_29 = BIT_INSERT_EXPR <x1.9_10, _28, 0>;

producing a vpblendw instruction instead of the expected vmovsh.  That's
either a missed vec_perm_const expansion optimization or even better,
an improvement - Zen4 for example has 4 ports to execute vpblendw
but only 3 for executing vmovsh and both instructions have the same size.

The patch XFAILs the sub-testcase.

	PR tree-optimization/94864
	PR tree-optimization/94865
	PR tree-optimization/93080
	* match.pd (bit_insert @0 (BIT_FIELD_REF @1 ..) ..): New pattern
	for vector insertion from vector extraction.

	* gcc.target/i386/pr94864.c: New testcase.
	* gcc.target/i386/pr94865.c: Likewise.
	* gcc.target/i386/avx512fp16-vmovsh-1a.c: XFAIL.
	* gcc.dg/tree-ssa/forwprop-40.c: Likewise.
	* gcc.dg/tree-ssa/forwprop-41.c: Likewise.