Revert rL333106 / D46814: [InstCombine] Fold unfolded masked merge pattern with variable mask!

In post-commit review, Eric Christopher notes that many
new MSan warnings are being observed with this patch.

The probable reason is: if 'y' is undef here and we could
evaluate it twice and get different results.
We can't increase the number of uses of a value.

llvm-svn: 333631