[IR] Revert r2694 in BasicBlock::removePredecessor

r2694 fixed a bug where removePredecessor could create IR with a use not
dominated by its def in a self loop. But this could only happen in an
unreachable loop, and since that time the rules have been relaxed so
that defs don't have to dominate uses in unreachable code, so the fix is
unnecessary. The regression test added in r2691 still stands.

Differential Revision: https://reviews.llvm.org/D80128