[SCCP] Fix crash when trying to constant-fold terminators multiple times.

If we fold a branch/switch to an unconditional branch to another dead block we
replace the branch with unreachable, to avoid attempting to fold the
unconditional branch.

Reviewers: davide, efriedma, mssimpso, jdoerfert

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D61300

llvm-svn: 360232