[X86] Don't fold non-LSB extracts into truncating broadcasts.

We used to incorrectly assume that the offset we're extracting from
was a multiple of the element size. So, we'd fold:
  (v8i16 (shufflevector
    (v8i16 (bitcast (v4i32 (build_vector X, Y, ...)))),
    <1,1,...,1>))
into:
  (v8i16 (vbroadcast (i16 (trunc Y))))
whereas we should have extracted the higher bits from X.

Instead, bail out if the assumption doesn't hold.

llvm-svn: 252361