i386: Support partial signbit/xorsign/copysign/abs/neg/and/xor/ior/andn for V2BF/V4BF

This patch adds support for bf16 operations in V2BF and V4BF modes on i386,
handling signbit, xorsign, copysign, abs, neg, and various logical operations.

gcc/ChangeLog:

	* config/i386/i386.cc (ix86_build_const_vector): Add V2BF/V4BF.
	(ix86_build_signbit_mask): Add V2BF/V4BF.
	* config/i386/mmx.md: Modified supported logic op to use VHBF_32_64.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/part-vect-absnegbf.c: New test.