i386: Introduce V2QImode minmax, abs and uavgv2hi3_ceil [PR103861]

Add V2QImode minmax, abs and uavxv2qi3_ceil operations with SSE registers.

2022-01-05  Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog:

	PR target/103861
	* config/i386/mmx.md (VI_16_32): New mode iterator.
	(VI1_16_32): Ditto.
	(mmxvecsize): Handle V2QI mode.
	(<smaxmin:code><mode>3): Rename from <smaxmin:code>v4qi3.
	Use VI1_16_32 mode iterator.
	(<umaxmin:code><mode>3): Rename from <umaxmin:code>v4qi3.
	Use VI1_16_32 mode iterator.
	(abs<mode>2): Use VI_16_32 mode iterator.
	(uavgv2qi3_ceil): New insn pattern.

gcc/testsuite/ChangeLog:

	PR target/103861
	* gcc.target/i386/pr103861-3.c: New test.
	* g++.dg/vect/slp-pr98855.cc (dg-final): Check that
	no vectorization using SLP was performed.