[ScalarizeMaskedMemIntrin] When expanding masked gathers, start with the passthru vector and insert the new load results into it.

Previously we started with undef and did a final merge with the passthru at the end.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343273 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed