Revert "[BasicAA] Remove unneeded special case for malloc/calloc"

This reverts commit 9b1e007.

Nikic reported in commit thread that I had forgotten history here, and that a) we'd tried this before, and b) had to revert due to an unexpected codegen impact.  Current measurements confirm the same issue still exists.

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -994,6 +994,20 @@ ModRefInfo BasicAAResult::getModRefInfo(const CallBase *Call,
     }
   }
 
+  // If the call is malloc/calloc like, we can assume that it doesn't
+  // modify any IR visible value.  This is only valid because we assume these
+  // routines do not read values visible in the IR.  TODO: Consider special
+  // casing realloc and strdup routines which access only their arguments as
+  // well.  Or alternatively, replace all of this with inaccessiblememonly once
+  // that's implemented fully.
+  if (isMallocOrCallocLikeFn(Call, &TLI)) {
+    // Be conservative if the accessed pointer may alias the allocation -
+    // fallback to the generic handling below.
+    if (getBestAAResults().alias(MemoryLocation::getBeforeOrAfter(Call), Loc,
+                                 AAQI) == AliasResult::NoAlias)
+      return ModRefInfo::NoModRef;
+  }
+
   // Ideally, there should be no need to special case for memcpy/memove
   // intrinsics here since general machinery (based on memory attributes) should
   // already handle it just fine. Unfortunately, it doesn't due to deficiency in

llvm/test/Transforms/GVN/nonescaping-malloc.ll

@@ -35,7 +35,7 @@ entry:
 
 declare i64 @strlen(i8* nocapture) nounwind readonly
 
-declare noalias i8* @malloc(i64) nounwind inaccessiblememonly
+declare noalias i8* @malloc(i64) nounwind
 
 declare i32 @_ZN4llvm13StringMapImpl15LookupBucketForENS_9StringRefE(%"struct.llvm::StringMapImpl"*, i64, i64)