Rename <sse2_avx2>_<insn><mode>3<mask_name> expander to a standard ssadd,
usadd, sssub and ussub name to enable corresponding optab expansion.

Also add named expander for MMX modes.

    PR middle-end/112600

gcc/ChangeLog:

    * config/i386/mmx.md (<insn><mode>3): New expander.
    * config/i386/sse.md
    (<sse2_avx2>_<sat_plusminus:insn><mode>3<mask_name>):
    Rename expander to <sat_plusminus:insn><mode>3<mask_name>.
    (<umaxmin:code><mode>3): Update for rename.
    * config/i386/i386-builtin.def (BDESC): Update for rename.

gcc/testsuite/ChangeLog:

    * gcc.target/i386/pr112600-1a.c: New test.
    * gcc.target/i386/pr112600-1b.c: New test.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Uros.