[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
diff --git a/llvm/unittests/Transforms/Utils/Local.cpp b/llvm/unittests/Transforms/Utils/Local.cpp
index bb36d01..9dc920d 100644
--- a/llvm/unittests/Transforms/Utils/Local.cpp
+++ b/llvm/unittests/Transforms/Utils/Local.cpp
@@ -552,7 +552,7 @@
       auto DI = dyn_cast<DbgValueInst>(&I);
       if (!DI) {
         // The function should only contain debug values and a terminator.
-        ASSERT_TRUE(isa<TerminatorInst>(&I));
+        ASSERT_TRUE(I.isTerminator());
         continue;
       }
       EXPECT_EQ(DI->getVariable()->getName(), "x");