[SLP][NFC] Precommit test that exposes a bug in ShuffleBuilder.

ShuffleBuilder generates a zero mask here:
`[[TMP6:%.*]] = shufflevector <2 x float> [[TMP3]], <2 x float> poison, <4 x i32> zeroinitializer`
But the correct mask is `0,0,1,1`, or we should have reused `TMP4`.

Differential Revision: https://reviews.llvm.org/D152868