[TI removal] Remove `TerminatorInst` from BasicBlockUtils.h

This requires updating a number of .cpp files to adapt to the new API.
I've just systematically updated all uses of `TerminatorInst` within
these files te `Instruction` so thta I won't have to touch them again in
the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344498 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed