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.

llvm-svn: 344566