[X86][SSE] Generalize the number of decoded shuffle inputs. NFCI.

combineX86ShufflesRecursively can still only handle a maximum of 2 shuffle inputs but everything before it now supports any number of shuffle inputs.

This will be necessary for combining OR(SHUFFLE, SHUFFLE) patterns.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293560 91177308-0d34-0410-b5e6-96231b3b80d8