[IR] Clean up dead instructions after simplifying a conditional branch

Change BasicBlock::removePredecessor to optionally return a vector of
instructions which might be dead. Use this in ConstantFoldTerminator to
delete them if they are dead.

Reapply with a bug fix: don't drop the "!KeepOneInputPHIs" argument when
removePredecessor calls PHINode::removeIncomingValue.

Differential Revision: https://reviews.llvm.org/D80206