i386: Add AVX512 support for STV of SI/DImode rotation by constant.

i386: Add AVX512 support for STV of SI/DImode rotation by constant.

Following Uros' suggestion, this patch adds support for AVX512VL's
vpro[lr][dq] instructions to the recently added scalar-to-vector (STV)
enhancements to handle DImode and SImode rotations by a constant.

For the test cases:

unsigned long long rot1(unsigned long long x) {
  return (x>>1) | (x<<63);
}

void mem1(unsigned long long *p) {
  *p = rot1(*p);
}

with -m32 -O2 -mavx512vl, we currently generate:

rot1:   movl    4(%esp), %eax
        movl    8(%esp), %edx
        movl    %eax, %ecx
        shrdl   $1, %edx, %eax
        shrdl   $1, %ecx, %edx
        ret

mem1:   movl    4(%esp), %eax
        vmovq   (%eax), %xmm0
        vpshufd $20, %xmm0, %xmm0
        vpsrlq  $1, %xmm0, %xmm0
        vpshufd $136, %xmm0, %xmm0
        vmovq   %xmm0, (%eax)
        ret

with this patch, we now generate:

rot1:   vmovq   4(%esp), %xmm0
        vprorq  $1, %xmm0, %xmm0
        vmovd   %xmm0, %eax
        vpextrd $1, %xmm0, %edx
        ret

mem1:	movl    4(%esp), %eax
        vmovq   (%eax), %xmm0
        vprorq  $1, %xmm0, %xmm0
        vmovq   %xmm0, (%eax)
        ret

2023-07-10  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-features.cc (compute_convert_gain): Tweak
	gains/costs for ROTATE/ROTATERT by integer constant on AVX512VL.
	(general_scalar_chain::convert_rotate): On TARGET_AVX512F generate
	avx512vl_rolv2di or avx412vl_rolv4si when appropriate.

gcc/testsuite/ChangeLog
	* gcc.target/i386/avx512vl-stv-rotatedi-1.c: New test case.