[X86][SSE] Fixed issue with commutation of 'faux unary' target shuffles (PR26667)

Fixed a bug introduced by D16683 when a binary shuffle is simplified to a unary shuffle (with undef/zero sentinel mask indices) - if this resulted in only the second input being used combineX86ShuffleChain failed to take this into account and still referenced the first input.

llvm-svn: 261434