[X86][SSE] Prefer trunc(movd(x)) to pextrb(x,0)

If we're extracting the 0'th index of a v16i8 vector we're better off using MOVD than PEXTRB, unless we're storing the value or we require the implicit zero extension of PEXTRB.

The biggest perf diff is on SLM targets where MOVD (uops=1, lat=3 tp=1) is notably faster than PEXTRB (uops=2, lat=5, tp=4).

This matches what we already do for PEXTRW.

Differential Revision: https://reviews.llvm.org/D76138