[X86][SSE] Improve PSHUFB shuffle mask decoding.

In cases where the PSHUFB shuffle mask is shared it might not be bitcasted to a vXi8 byte vector. This patch adds support for decoding these wider shuffle masks from the ConstantPool.

The test case in question makes use of this to recognise the shuffle mask is an unary UNPCKL pattern and simplifies accordingly.

llvm-svn: 261201