[X86] Use VEX_WIG for VPINSRB/W and VPEXTRB/W to match what is done for EVEX.

The instruction's document this as W0 for the VEX encoding. But there's a
footnote mentioning that VEX.W is ignored in 64-bit mode. And the main VEX
encoding description says the VEX.W bit is ignored for instructions that are
equivalent to a legacy SSE instruction that uses REX.W to select a GPR which
would apply here.

By making this match EVEX we can remove a special case of allowing EVEX2VEX to
turn an EVEX.WIG instruction into VEX.W0.

llvm-svn: 357971