revert r317812 [BasicAA] fix build break by converting the previously introduced assert into an if stmt

The code has a bug, but some tests regress.
I'll discuss this further on the mailing list.

llvm-svn: 317815
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 528806a..fb9ece2 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1289,8 +1289,7 @@
   // greater, we know they do not overlap.
   if (GEP1BaseOffset != 0 && DecompGEP1.VarIndices.empty()) {
     if (GEP1BaseOffset >= 0) {
-      if (V1Size != MemoryLocation::UnknownSize &&
-          V2Size != MemoryLocation::UnknownSize) {
+      if (V2Size != MemoryLocation::UnknownSize) {
         if ((uint64_t)GEP1BaseOffset < V2Size)
           return PartialAlias;
         return NoAlias;