[X86] Remove X86ISD::SHUF128 from combineBitcastForMaskedOp. Use isel patterns instead.

We always created X86ISD::SHUF128 with a 64-bit element type so we can use isel patterns to detect a bitconvert to 32-bit to handle masking.

The test changes are because we also match the bitconvert even if there is no masking. This leads to unnecessary isel pattern, but it requires more multiclass hackery in tablegen to get rid of it.

llvm-svn: 324205