[X86][XOP] Fix VPERMIL2 non-constant pool shuffle decoding (PR31296)

The non-constant pool version of DecodeVPERMIL2PMask was not offsetting correctly for the second input. I've updated the code to match the implementation in the constant-pool version.

Annoyingly this bug was hidden for so long as it's tricky to combine to useful variable shuffle masks that don't become constant-pool entries.

llvm-svn: 288898