i386: Introduce V2QImode vectorized shifts [PR103861]

Add V2QImode shift operations and split them to synthesized
double HI/LO QImode operations with integer registers.

Also robustify arithmetic split patterns.

2022-01-13  Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog:

	PR target/103861
	* config/i386/i386.md (*ashlqi_ext<mode>_2): New insn pattern.
	(*<any_shiftrt:insn>qi_ext<mode>_2): Ditto.
	* config/i386/mmx.md (<any_shift:insn>v2qi):
	New insn_and_split pattern.

gcc/testsuite/ChangeLog:

	PR target/103861
	* gcc.target/i386/pr103861.c (shl,ashr,lshr): New tests.