Emulate MMX packsswb/packssdw/packuswb with SSE packsswb/packssdw/packuswb
plus moving bits 64:95 to bits 32:63 in SSE register.  Only SSE register
source operand is allowed.

	PR target/89021
	* config/i386/i386-expand.c (ix86_move_vector_high_sse_to_mmx):
	New function.
	(ix86_split_mmx_pack): Likewise.
	* config/i386/i386-protos.h (ix86_move_vector_high_sse_to_mmx):
	New prototype.
	(ix86_split_mmx_pack): Likewise.
	* config/i386/i386.md (mmx_isa): New.
	(enabled): Also check mmx_isa.
	* config/i386/mmx.md (any_s_truncate): New code iterator.
	(s_trunsuffix): New code attr.
	(mmx_packsswb): Removed.
	(mmx_packssdw): Likewise.
	(mmx_packuswb): Likewise.
	(mmx_pack<s_trunsuffix>swb): New define_insn_and_split to emulate
	MMX packsswb/packuswb with SSE2.
	(mmx_packssdw): Likewise.
	* config/i386/predicates.md (register_mmxmem_operand): New.

Co-Authored-By: Uros Bizjak <ubizjak@gmail.com>

From-SVN: r271215