[X86][SSE] Fixed PALIGNR target shuffle decode

The PALIGNR target shuffle decode was not taking into account that DecodePALIGNRMask (rather oddly) expects the operands to be in reverse order, nor was it detecting unary patterns, causing combines to combine with the incorrect input.

The cgbuiltin, auto upgrade and instruction comments code correctly swap the operands so are not affected.

llvm-svn: 278494