Reimplement packuswb/packusdw with UNSPEC_US_TRUNCATE instead of original us_truncate.

Reimplement packuswb/packusdw with UNSPEC_US_TRUNCATE instead of original us_truncate.

packuswb/packusdw does unsigned saturation for signed source, but rtl
us_truncate means does unsigned saturation for unsigned source.
So for value -1, packuswb will produce 0, but us_truncate produces
255. The patch reimplement those related patterns and functions with
UNSPEC_US_TRUNCATE instead of us_truncate.

gcc/ChangeLog:

	PR target/110235
	* config/i386/i386-expand.cc (ix86_split_mmx_pack): Use
	UNSPEC_US_TRUNCATE instead of original us_truncate for
	packusdw/packuswb.
	* config/i386/mmx.md (mmx_pack<s_trunsuffix>swb): Substitute
	with ..
	(mmx_packsswb): .. this and ..
	(mmx_packuswb): .. this.
	(mmx_packusdw): Use UNSPEC_US_TRUNCATE instead of original
	us_truncate.
	(s_trunsuffix): Removed code iterator.
	(any_s_truncate): Ditto.
	* config/i386/sse.md (<sse2_avx2>_packuswb<mask_name>): Use
	UNSPEC_US_TRUNCATE instead of original us_truncate.
	(<sse4_1_avx2>_packusdw<mask_name>): Ditto.
	* config/i386/i386.md (UNSPEC_US_TRUNCATE): New unspec_c_enum.