Revert "[IfConversion] Keep the CFG updated incrementally in IfConvertTriangle"

The reverted change introdued assertions ala:

"MachineBasicBlock::succ_iterator
llvm::MachineBasicBlock::removeSuccessor(succ_iterator, bool): Assertion
`I != Successors.end() && "Not a current successor!"'

Mikael, the original committer, wrote me that he is working on a fix, but that
it likely will take some time to get this resolved. As this bug is one of the
last two issues that keep the AOSP buildbot from turning green, I revert the
original commit r302876.

I am looking forward to see this recommitted after the assertion has been
resolved.

llvm-svn: 304128