rs6000: Replace UNSPECS with ss_plus/us_plus and ss_minus/us_minus

rs6000: Replace UNSPECS with ss_plus/us_plus and ss_minus/us_minus

These four UNSPECS seems could be replaced with native RTL.

For
"(set (reg:SI VSCR_REGNO) (unspec:SI [(const_int 0)] UNSPEC_SET_VSCR))":

Quoted David's explanation:

"The design came from the early implementation of Altivec:

https://gcc.gnu.org/pipermail/gcc-patches/2002-May/077409.html

If one later checks for saturation (reads VSCR), one needs a
corresponding SET of the value.  It's set in an architecture-specific
manner that isn't described to GCC, but it's set, not just clobbered
and in an undefined state.

The RTL does not describe that VSCR is set to the value 0.  The
(const_int 0) is not the value set.  You can think of the (const_int
0) as a dummy RTL argument to the VSCR UNSPEC.  UNSPEC requires at
least one argument and the pattern doesn't try to express the
argument, so it uses a dummy RTL constant.  It's part of a PARALLEL
and the plus or minus already expresses the data dependency of the
pattern on the input operands."

gcc/ChangeLog:

2021-12-21  Xionghu Luo  <luoxhu@linux.ibm.com>

	* config/rs6000/altivec.md (altivec_vaddu<VI_char>s): Replace
	UNSPEC_VADDU with us_plus.
	(altivec_vadds<VI_char>s): Replace UNSPEC_VADDS with ss_plus.
	(altivec_vsubu<VI_char>s): Replace UNSPEC_VSUBU with us_minus.
	(altivec_vsubs<VI_char>s): Replace UNSPEC_VSUBS with ss_minus.
	(altivec_abss_<mode>): Likewise.