BranchFolder: Fix invalid undef flags after merge.

It is legal to merge instructions with different undef flags; However we
must drop the undef flag from the merged instruction if it isn't present
everywhere.

This fixes http://llvm.org/PR30199

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281957 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed