i386: Add V8QI and V4QImode partial vector shift operations

i386: Add V8QI and V4QImode partial vector shift operations

Add V8QImode and V4QImode vector shift patterns that call into
ix86_expand_vecop_qihi_partial.  Generate special sequences
for constant count operands.

gcc/ChangeLog:

	* config/i386/i386-expand.cc (ix86_expand_vecop_qihi_partial):
	Call ix86_expand_vec_shift_qihi_constant for shifts
	with constant count operand.
	* config/i386/i386.cc (ix86_shift_rotate_cost):
	Handle V4QImode and V8QImode.
	* config/i386/mmx.md (<insn>v8qi3): New insn pattern.
	(<insn>v4qi3): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/vect-shiftv4qi.c: New test.
	* gcc.target/i386/vect-shiftv8qi.c: New test.