[RS4GC] Update and simplify handling of Constants in findBaseDefiningValueOfVector().

[mjacob]

Summary

This is analogous to r256079, which removed an overly strong assertion, and
r256812, which simplified the code by replacing three conditionals by one.

[reames]

LGTM w/a test case or two added.

[mjacob]

Add test case.

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

findBaseDefiningValueOfVector(Value *I) {
 
   // Each case parallels findBaseDefiningValue below, see that code for
   // detailed motivation.
 
   if (isa<Argument>(I))
     // An incoming argument to the function is a base pointer
     return BaseDefiningValueResult(I, true);
 
-  // We shouldn't see the address of a global as a vector value?
-  assert(!isa<GlobalVariable>(I) &&
-         "unexpected global variable found in base of vector");
-
-  // 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.
+  if (isa<Constant>(I))
+    // Constant vectors consist only of constant pointers.
     return BaseDefiningValueResult(I, true);
 
-  // Due to inheritance, this must be _after_ the global variable and undef
-  // checks
-  if (Constant *Con = dyn_cast<Constant>(I)) {
-    assert(!isa<GlobalVariable>(I) && !isa<UndefValue>(I) &&
-           "order of checks wrong!");
-    assert(Con->isNullValue() && "null is the only case which makes sense");
-    return BaseDefiningValueResult(Con, true);
-  }
-  
   if (isa<LoadInst>(I))
     return BaseDefiningValueResult(I, true);
 
   if (isa<InsertElementInst>(I))
     // We don't know whether this vector contains entirely base pointers or
     // not.  To be conservatively correct, we treat it as a BDV and will
     // duplicate code as needed to construct a parallel vector of bases.
     return BaseDefiningValueResult(I, false);