[MemCpyOpt] Don't unconditionally skip modref visitation for passthru

Previously passthrus never had effects. Now they do, if it it's
a call with a ret-only capture. Always run the modref handling
code, but wrap it in a quick read/write check to avoid wasting
time for cases that definitely don't modref.

Author: nikic

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -1552,30 +1552,32 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
         UseCaptureInfo CI =
             DetermineUseCaptureKind(U, AI, IsDereferenceableOrNull);
         // TODO(captures): Make this more precise.
-        if (CI.isPassthrough()) {
-          Worklist.push_back(UI);
-          continue;
-        }
-
-        if (capturesAnything(CI))
+        if (capturesAnything(CI.UseCC))
           return false;
 
-        if (UI->isLifetimeStartOrEnd()) {
-          // We note the locations of these intrinsic calls so that we can
-          // delete them later if the optimization succeeds, this is safe
-          // since both llvm.lifetime.start and llvm.lifetime.end intrinsics
-          // practically fill all the bytes of the alloca with an undefined
-          // value, although conceptually marked as alive/dead.
-          int64_t Size = cast<ConstantInt>(UI->getOperand(0))->getSExtValue();
-          if (Size < 0 || Size == DestSize) {
-            LifetimeMarkers.push_back(UI);
-            continue;
+        if (UI->mayReadOrWriteMemory()) {
+          if (UI->isLifetimeStartOrEnd()) {
+            // We note the locations of these intrinsic calls so that we can
+            // delete them later if the optimization succeeds, this is safe
+            // since both llvm.lifetime.start and llvm.lifetime.end intrinsics
+            // practically fill all the bytes of the alloca with an undefined
+            // value, although conceptually marked as alive/dead.
+            int64_t Size = cast<ConstantInt>(UI->getOperand(0))->getSExtValue();
+            if (Size < 0 || Size == DestSize) {
+              LifetimeMarkers.push_back(UI);
+              continue;
+            }
           }
+          if (UI->hasMetadata(LLVMContext::MD_noalias))
+            NoAliasInstrs.insert(UI);
+          if (!ModRefCallback(UI))
+            return false;
+        }
+
+        if (capturesAnything(CI.ResultCC)) {
+          Worklist.push_back(UI);
+          continue;
         }
-        if (UI->hasMetadata(LLVMContext::MD_noalias))
-          NoAliasInstrs.insert(UI);
-        if (!ModRefCallback(UI))
-          return false;
       }
     }
     return true;