[InstCombine] Fold unfolded masked merge pattern with variable mask!

Summary:
Finally fixes [[ https://bugs.llvm.org/show_bug.cgi?id=6773 | PR6773 ]].

Now that the backend is all done, we can finally fold it!

The canonical unfolded masked merge pattern is
```(x &  m) | (y & ~m)```
There is a second, equivalent variant:
```(x | ~m) & (y |  m)```
Only one of them (the or-of-and's i think) is canonical.
And if the mask is not a constant, we should fold it to:
```((x ^ y) & M) ^ y```

https://rise4fun.com/Alive/ndQw

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: nicholas, RKSimon, llvm-commits

Differential Revision: https://reviews.llvm.org/D46814

llvm-svn: 333106