Support vector float_truncate for SF to BF.

Support vector float_truncate for SF to BF.

Generate native instruction whenever possible, otherwise use vector
permutation with odd indices.

gcc/ChangeLog:

	* config/i386/i386-expand.cc
	(ix86_expand_vector_sf2bf_with_vec_perm): New function.
	* config/i386/i386-protos.h
	(ix86_expand_vector_sf2bf_with_vec_perm): New declare.
	* config/i386/mmx.md (truncv2sfv2bf2): New expander.
	* config/i386/sse.md (truncv4sfv4bf2): Ditto.
	(truncv8sfv8bf2): Ditto.
	(truncv16sfv16bf2): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512bf16-truncsfbf.c: New test.
	* gcc.target/i386/avx512bw-truncsfbf.c: New test.
	* gcc.target/i386/ssse3-truncsfbf.c: New test.