x86: make VPTERNLOG* usable on less than 512-bit operands with just AVX512F

x86: make VPTERNLOG* usable on less than 512-bit operands with just AVX512F

There's no reason to constrain this to AVX512VL, unless instructed so by
-mprefer-vector-width=, as the wider operation is unusable for more
narrow operands only when the possible memory source is a non-broadcast
one. This way even the scalar copysign<mode>3 can benefit from the
operation being a single-insn one (leaving aside moves which the
compiler decides to insert for unclear reasons, and leaving aside the
fact that bcst_mem_operand() is too restrictive for broadcast to be
embedded right into VPTERNLOG*).

While there also bring *<avx512>_vternlog<mode>_all's in sync with that
of the three splitters.

Along with this also request value duplication in
ix86_expand_copysign()'s call to ix86_build_signbit_mask(), eliminating
excess space allocation in .rodata.*, filled with zeros which are never
read.

gcc/

	* config/i386/i386-expand.cc (ix86_expand_copysign): Request
	value duplication by ix86_build_signbit_mask() when AVX512F and
	not HFmode.
	* config/i386/sse.md (*<avx512>_vternlog<mode>_all): Convert to
	2-alternative form. Adjust "mode" attribute. Add "enabled"
	attribute.
	(*<avx512>_vpternlog<mode>_1): Also permit when TARGET_AVX512F
	&& !TARGET_PREFER_AVX256.
	(*<avx512>_vpternlog<mode>_2): Likewise.
	(*<avx512>_vpternlog<mode>_3): Likewise.

gcc/testsuite/

	* gcc.target/i386/avx512f-copysign.c: New test.