Add UNSPEC_MASKOP to kupck<mode> instructions in sse.md on x86.

Add UNSPEC_MASKOP to kupck<mode> instructions in sse.md on x86.

This AVX512 specific patch to sse.md is split out from an earlier patch:
https://gcc.gnu.org/pipermail/gcc-patches/2022-June/596199.html

The new splitters proposed in that patch interfere with AVX512's
kunpckdq instruction which is defined as identical RTL,
DW:DI = (HI:SI<<32)|zero_extend(LO:SI).  To distinguish these,
and avoid AVX512 mask registers accidentally being (ab)used by reload
to perform SImode scalar shifts, this patch adds the explicit
(unspec UNSPEC_MASKOP) to the unpack mask operations, which matches
what sse.md does for the other mask specific (logic) operations.

2022-07-18  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/sse.md (kunpckhi): Add UNSPEC_MASKOP unspec.
	(kunpcksi): Likewise, add UNSPEC_MASKOP unspec.
	(kunpckdi): Likewise, add UNSPEC_MASKOP unspec.
	(vec_pack_trunc_qi): Update to specify the now required
	UNSPEC_MASKOP unspec.
	(vec_pack_trunc_<mode>): Likewise.