[X86] Emit a single shuffle for the v16i8->v4i32 step of a...

[X86] Emit a single shuffle for the v16i8->v4i32 step of a SIGN_EXTEND_VECTOR_INREG lowering on pre-sse4.1 targets.

Previously we emitted to separate shuffles, one for unpcklbw and one for unpcklwd. Instead emit a single shuffle equivalent to both of the original shuffles. Shuffle lowering seems able to handle it. This avoids a bitcast between the two shuffles which seems helpful to DAG combine.

Remove the custom type legalization for v8i8->v8i32. I had put that in to avoid some almost duplicate punpcklbw instructions I was seeing, but this lowering change seems to fix that. It also fixes some duplicate shuffles seen in vector-sext.ll

llvm-svn: 347348