[BasicAA] Update comments based on feedback from hfinkel. NFCI.

Original change Hal's comments were based on:
http://reviews.llvm.org/D19730

llvm-svn: 269678

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -570,6 +570,9 @@ FunctionModRefBehavior BasicAAResult::getModRefBehavior(ImmutableCallSite CS) {
 
 /// Returns the behavior when calling the given function. For use when the call
 /// site is not known.
+/// NOTE: Because of the special case handling of llvm.assume below, the result
+/// of this function may not match similar results derived from function
+/// attributes (e.g. "readnone").
 FunctionModRefBehavior BasicAAResult::getModRefBehavior(const Function *F) {
   // If the function declares it doesn't access memory, we can't do better.
   if (F->doesNotAccessMemory())
@@ -577,7 +580,7 @@ FunctionModRefBehavior BasicAAResult::getModRefBehavior(const Function *F) {
 
   // While the assume intrinsic is marked as arbitrarily writing so that
   // proper control dependencies will be maintained, it never aliases any
-  // particular memory location.
+  // actual memory locations.
   if (F->getIntrinsicID() == Intrinsic::assume)
     return FMRB_DoesNotAccessMemory;