x86: Support 2/4/8 byte constant vector stores

x86: Support 2/4/8 byte constant vector stores

1. Add a predicate for constant vectors which can be converted to integer
constants suitable for constant integer stores.  For a 8-byte constant
vector, the converted 64-bit integer must be valid for store with 64-bit
immediate, which is a 64-bit integer sign-extended from a 32-bit integer.
2. Add a new pattern to allow 2-byte, 4-byte and 8-byte constant vector
stores, like

(set (mem:V2HI (reg:DI 84))
     (const_vector:V2HI [(const_int 0 [0]) (const_int 1 [0x1])]))

3. After reload, convert constant vector stores to constant integer
stores, like

(set (mem:SI (reg:DI 5 di [84]))
     (const_int 65536 [0x10000]))

For

void
foo (short * c)
{
  c[0] = 0;
  c[1] = 1;
}

it generates

	movl	$65536, (%rdi)

instead of

	movl	.LC0(%rip), %eax
	movl	%eax, (%rdi)

gcc/

	PR target/106022
	* config/i386/i386-protos.h (ix86_convert_const_vector_to_integer):
	New.
	* config/i386/i386.cc (ix86_convert_const_vector_to_integer):
	New.
	* config/i386/mmx.md (V_16_32_64): New.
	(*mov<mode>_imm): New patterns for stores with 16-bit, 32-bit
	and 64-bit constant vector.
	* config/i386/predicates.md (x86_64_const_vector_operand): New.

gcc/testsuite/

	PR target/106022
	* gcc.target/i386/pr106022-1.c: New test.
	* gcc.target/i386/pr106022-2.c: Likewise.
	* gcc.target/i386/pr106022-3.c: Likewise.
	* gcc.target/i386/pr106022-4.c: Likewise.