[X86][SSE] getFauxShuffleMask - Fix shuffle mask adjustment for multiple inserted subvectors

Part of the issue discovered in PR39483, although its not fully exposed until I reapply rL345395 (by reverting rL345451)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345520 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed