i386: Use 2x-wider modes when emulating QImode vector instructions

i386: Use 2x-wider modes when emulating QImode vector instructions

Rewrite ix86_expand_vecop_qihi2 to expand fo 2x-wider (e.g. V16QI -> V16HImode)
instructions when available.  Currently, the compiler generates following
assembly for V16QImode multiplication (-mavx2):

	vpunpcklbw      %xmm0, %xmm0, %xmm3
	vpunpcklbw      %xmm1, %xmm1, %xmm2
	vpunpckhbw      %xmm0, %xmm0, %xmm0
	movl    $255, %eax
	vpunpckhbw      %xmm1, %xmm1, %xmm1
	vpmullw %xmm3, %xmm2, %xmm2
	vmovd   %eax, %xmm3
	vpmullw %xmm0, %xmm1, %xmm1
	vpbroadcastw    %xmm3, %xmm3
	vpand   %xmm2, %xmm3, %xmm0
	vpand   %xmm1, %xmm3, %xmm3
	vpackuswb       %xmm3, %xmm0, %xmm0

and only with -mavx512bw -mavx512vl generates:

	vpmovzxbw       %xmm1, %ymm1
	vpmovzxbw       %xmm0, %ymm0
	vpmullw %ymm1, %ymm0, %ymm0
	vpmovwb %ymm0, %xmm0

Patched compiler generates more optimized code involving multiplication
in 2x-wider mode in cases where missing truncate instruction has to be
emulated with a permutation (-mavx2):

	vpmovzxbw       %xmm0, %ymm0
	vpmovzxbw       %xmm1, %ymm1
	movl    $255, %eax
	vpmullw %ymm1, %ymm0, %ymm1
	vmovd   %eax, %xmm0
	vpbroadcastw    %xmm0, %ymm0
	vpand   %ymm1, %ymm0, %ymm0
	vpackuswb       %ymm0, %ymm0, %ymm0
	vpermq  $216, %ymm0, %ymm0

The patch also adjusts cost calculation of V*QImode emulations to account
for generation of 2x-wider mode instructions.

gcc/ChangeLog:

	* config/i386/i386-expand.cc (ix86_expand_vecop_qihi2):
	Rewrite to expand to 2x-wider (e.g. V16QI -> V16HImode)
	instructions when available.  Emulate truncation via
	ix86_expand_vec_perm_const_1 when native truncate insn
	is not available.
	(ix86_expand_vecop_qihi_partial) <case MULT>: Use pmovzx
	when available.  Trivially rename some variables.
	(ix86_expand_vecop_qihi): Unconditionally call ix86_expand_vecop_qihi2.
	* config/i386/i386.cc (ix86_multiplication_cost): Rewrite cost
	calculation of V*QImode emulations to account for generation of
	2x-wider mode instructions.
	(ix86_shift_rotate_cost): Update cost calculation of V*QImode
	emulations to account for generation of 2x-wider mode instructions.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512vl-pr95488-1.c: Revert 2023-05-18 change.