Change a TerminatorInst* to an Instruction* in HotColdSplitting.cpp.

Change a TerminatorInst* to an Instruction* in HotColdSplitting.cpp.

r344558 added an assignment to a TerminatorInst* from
BasicBlock::getTerminatorInst(), but BasicBlock::getTerminatorInst() returns an
Instruction* rather than a TerminatorInst* since r344504 so this fails to
compile.

Changing the variable to an Instruction* should get the bots building again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344566 91177308-0d34-0410-b5e6-96231b3b80d8