[X86][AVX] Don't fold PEXTR(VBROADCAST_LOAD(X)) -> LOAD(X).

We were checking the VBROADCAST_LOAD element size against the extraction destination size instead of the extracted vector element size - PEXTRW/PEXTB have implicit zext'ing so have i32 destination sizes for v8i16/v16i8 vectors, resulting in us extracting from the wrong part of a load.

This patch bails from the fold if the vector element sizes don't match, and we now use the target constant extraction code later on like the pre-AVX2 targets, fixing the test case.

Found by internal fuzzing tests.