[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/lib/Analysis/MemoryDependenceAnalysis.cpp b/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
index feae53c..a0e22ad 100644
--- a/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -1572,7 +1572,7 @@
   ReverseDepMapType::iterator ReverseDepIt = ReverseLocalDeps.find(RemInst);
   if (ReverseDepIt != ReverseLocalDeps.end()) {
     // RemInst can't be the terminator if it has local stuff depending on it.
-    assert(!ReverseDepIt->second.empty() && !isa<TerminatorInst>(RemInst) &&
+    assert(!ReverseDepIt->second.empty() && !RemInst->isTerminator() &&
            "Nothing can locally depend on a terminator");
 
     for (Instruction *InstDependingOnRemInst : ReverseDepIt->second) {