[X86] In LowerTRUNCATE, create an ISD::VECTOR_SHUFFLE instead of explicitly...

[X86] In LowerTRUNCATE, create an ISD::VECTOR_SHUFFLE instead of explicitly creating a PSHUFB. This will be lowered by regular shuffle lowering to a PSHUFB later.

Similar was already done for several other shuffles in this function.

The test changes are because the old code used explicity zeroing for elements that could have been undef.

While I was here I also changed other shuffle vectors in the same function to use the same input twice instead of creating UNDEF nodes. getVectorShuffle can create the UNDEF for us.

llvm-svn: 294130