[SimplifyCFG] FoldTwoEntryPHINode(): don't bailout on i1 PHI's if we can hoist...

[SimplifyCFG] FoldTwoEntryPHINode(): don't bailout on i1 PHI's if we can hoist a 'not' from incoming values

Summary:
As it can be seen in the tests in D65143/D65144, even though we have formed an '@llvm.umul.with.overflow'
and got rid of potential for division-by-zero, the control flow remains, we still have that branch.

We have this condition:
```
  // Don't fold i1 branches on PHIs which contain binary operators
  // These can often be turned into switches and other things.
  if (PN->getType()->isIntegerTy(1) &&
      (isa<BinaryOperator>(PN->getIncomingValue(0)) ||
       isa<BinaryOperator>(PN->getIncomingValue(1)) ||
       isa<BinaryOperator>(IfCond)))
    return false;
```
which was added back in rL121764 to help with `select` formation i think?

That check prevents us to flatten the CFG here, even though we know
we no longer need that guard and will be able to drop everything
but the '@llvm.umul.with.overflow' + `not`.

As it can be seen from tests, we end here because the `not` is being
sinked into the PHI's incoming values by InstCombine,
so we can't workaround this by hoisting it to after PHI.

Thus i suggest that we relax that check to not bailout if we'd get to hoist the `not`.

Reviewers: craig.topper, spatel, fhahn, nikic

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65147

llvm-svn: 370349