[X86] Don't generate vbroadcasti128 for v4i64 splats from memory.

We used to erroneously match:
    (v4i64 shuffle (v2i64 load), <0,0,0,0>)

Whereas vbroadcasti128 is more like:
    (v4i64 shuffle (v2i64 load), <0,1,0,1>)

This problem doesn't exist for vbroadcastf128, which kept matching
the intrinsic after r231182.  We should perhaps re-introduce the
intrinsic here as well, but that's a separate issue still being
discussed.

While there, add some proper vbroadcastf128 tests.  We don't currently
match those, like for loading vbroadcastsd/ss on AVX (the reg-reg
broadcasts where added in AVX2).

Fixes PR23886.

llvm-svn: 240488