[RS4GC] Simplify handling of Constants in findBaseDefiningValue(). NFC.

[mjacob]

Summary

Previously there were three conditionals, checking for global
variables, undef values and everything constant except these two, all three
returning the same value. This commit replaces them by one conditional.

[reames]

LGTM

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

static BaseDefiningValueResult findBaseDefiningValue(Value *I) {
   assert(I->getType()->isPointerTy() &&
          "Illegal to ask for the base pointer of a non-pointer type");
 
   if (isa<Argument>(I))
     // An incoming argument to the function is a base pointer
     // We should have never reached here if this argument isn't an gc value
     return BaseDefiningValueResult(I, true);
 
-  if (isa<GlobalVariable>(I))
-    // base case
+  if (isa<Constant>(I))
+    // We assume that objects with a constant base (e.g. a global) can't move
+    // and don't need to be reported to the collector because they are always
+    // live.  All constants have constant bases.  Besides global references, all
+    // kinds of constants (e.g. undef, constant expressions, null pointers) can
+    // be introduced by the inliner or the optimizer, especially on dynamically
+    // dead paths.  See e.g. test4 in constants.ll.
     return BaseDefiningValueResult(I, true);
 
-  // inlining could possibly introduce phi node that contains
-  // undef if callee has multiple returns
-  if (isa<UndefValue>(I))
-    // utterly meaningless, but useful for dealing with
-    // partially optimized code.
-    return BaseDefiningValueResult(I, true);
-
-  // Due to inheritance, this must be _after_ the global variable and undef
-  // checks
-  if (isa<Constant>(I)) {
-    assert(!isa<GlobalVariable>(I) && !isa<UndefValue>(I) &&
-           "order of checks wrong!");
-    // Note: Even for frontends which don't have constant references, we can
-    // see constants appearing after optimizations.  A simple example is
-    // specialization of an address computation on null feeding into a merge
-    // point where the actual use of the now-constant input is protected by
-    // another null check.  (e.g. test4 in constants.ll)
-    return BaseDefiningValueResult(I, true);
-  }
-
   if (CastInst *CI = dyn_cast<CastInst>(I)) {
     Value *Def = CI->stripPointerCasts();
     // If stripping pointer casts changes the address space there is an
     // addrspacecast in between.
     assert(cast<PointerType>(Def->getType())->getAddressSpace() ==
                cast<PointerType>(CI->getType())->getAddressSpace() &&
            "unsupported addrspacecast");
     // If we find a cast instruction here, it means we've found a cast which is