i386: Add infrastructure for QImode partial vector mult and shift operations

i386: Add infrastructure for QImode partial vector mult and shift operations

QImode partial vector multiplications and shifts can be implemented using
their HImode counterparts.  Add infrastructure to handle V8QImode and
V4QImode vectors by extending (interleaving) their input operands to
V8HImode, performing V8HImode operation and truncating output back to
the original QImode vector.

The patch implements V8QImode and V4QImode multiplication for SSE2 targets,
using generic permutation to truncate output operand, but still taking
advantage of VPMOVWB down convert instruction, when available.

The patch also removes setting of REG_EQAUL note to the last insn
of ix86_expand_vecop_qihi expander.  This is what generic code does
automatically when named pattern is expanded.

gcc/ChangeLog:

	* config/i386/i386-expand.cc (ix86_expand_vecop_qihi_partial): New.
	(ix86_expand_vecop_qihi): Add op2vec bool variable.
	Do not set REG_EQUAL note.
	* config/i386/i386-protos.h (ix86_expand_vecop_qihi_partial):
	Add prototype.
	* config/i386/i386.cc (ix86_multiplication_cost): Handle
	V4QImode and V8QImode.
	* config/i386/mmx.md (mulv8qi3): New expander.
	(mulv4qi3): Ditto.
	* config/i386/sse.md (mulv8qi3): Remove.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512vl-pr95488-1.c: Adjust
	expected scan-assembler-times frequency and strings..
	* gcc.target/i386/vect-mulv4qi.c: New test.
	* gcc.target/i386/vect-mulv8qi.c: New test.