[IR] Replace isa<TerminatorInst> with isTerminator().

This is a bit awkward in a handful of places where we didn't even have
an instruction and now we have to see if we can build one. But on the
whole, this seems like a win and at worst a reasonable cost for removing
`TerminatorInst`.

All of this is part of the removal of `TerminatorInst` from the
`Instruction` type hierarchy.

llvm-svn: 340701

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -4620,7 +4620,7 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
     CurBB->getInstList().push_back(I);
 
     // If this was a terminator instruction, move to the next block.
-    if (isa<TerminatorInst>(I)) {
+    if (I->isTerminator()) {
       ++CurBBNo;
       CurBB = CurBBNo < FunctionBBs.size() ? FunctionBBs[CurBBNo] : nullptr;
     }