Revert "[SimplifyCFG] Handle branch on same condition in pred more directly"...

Revert "[SimplifyCFG] Handle branch on same condition in pred more directly" and "[SimplifyCFG] Make FoldCondBranchOnPHI more amenable to extension"

This reverts commit 3df86e79.
This reverts commit 89882546.

`[SimplifyCFG] Handle branch on same condition in pred more directly`
caused non-determinism when compiling opt with a bootstrapped clang.
I have to revert the dependent commit as well.