x86: Optimize load of const FP all bits set vectors

x86: Optimize load of const FP all bits set vectors

Check float_vector_all_ones_operand for vector floating-point modes to
optimize load of const floating-point all bits set vectors.

gcc/

	PR target/101804
	* config/i386/constraints.md (BC): Document for integer SSE
	constant all bits set operand.
	(BF): New constraint for const floating-point all bits set
	vectors.
	* config/i386/i386.c (standard_sse_constant_p): Likewise.
	(standard_sse_constant_opcode): Likewise.
	* config/i386/sse.md (sseconstm1): New mode attribute.
	(mov<mode>_internal): Replace BC with <sseconstm1>.

gcc/testsuite/

	PR target/101804
	* gcc.target/i386/avx2-gather-2.c: Pass -march=skylake instead
	of "-mavx2 -mtune=skylake".  Scan vpcmpeqd.