[X86][SSE] Move 2-input limit up from getFauxShuffleMask to resolveTargetShuffleInputs (reapplied)

Reapplying an updated version of rL345395 (reverted in rL345451), now the issues noticed in PR39483 have been fixed. 

This patch allows resolveTargetShuffleInputs to remove UNDEF inputs from cases where we have more than 2 inputs.

llvm-svn: 345824