[DSE] Make capture check more precise

It is sufficient that the object has not been captured before the
load that produces the pointer we're loading. A capture after that
can not affect the already loaded pointer.

This is small part of D110368 applied separately.

Author: nikic

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -1318,10 +1318,10 @@ struct DSEState {
       // before the load.
       auto *ReadUO = getUnderlyingObject(LI->getPointerOperand());
       auto *DefUO = getUnderlyingObject(DefLoc.Ptr);
-      if (DefUO && ReadUO && isa<LoadInst>(ReadUO) &&
-          notCapturedBeforeOrAt(DefUO, UseInst)) {
+      auto *ReadLI = dyn_cast<LoadInst>(ReadUO);
+      if (ReadLI && notCapturedBeforeOrAt(DefUO, ReadLI)) {
         assert(
-            !PointerMayBeCapturedBefore(DefLoc.Ptr, false, true, UseInst, &DT,
+            !PointerMayBeCapturedBefore(DefLoc.Ptr, false, true, ReadLI, &DT,
                                         false, 0, &this->LI) &&
             "cached analysis disagrees with fresh PointerMayBeCapturedBefore");
         return false;

llvm/test/Transforms/DeadStoreElimination/captures-before-load.ll

@@ -113,7 +113,6 @@ define i32 @test_captured_and_clobbered_before_load_same_bb_1(i32** %in.ptr) {
 define i32 @test_captured_before_load_same_bb_1_clobbered_later(i32** %in.ptr) {
 ; CHECK-LABEL: @test_captured_before_load_same_bb_1_clobbered_later(
 ; CHECK-NEXT:    [[A:%.*]] = alloca i32, align 4
-; CHECK-NEXT:    store i32 55, i32* [[A]], align 4
 ; CHECK-NEXT:    [[IN_LV_1:%.*]] = load i32*, i32** [[IN_PTR:%.*]], align 2
 ; CHECK-NEXT:    call void @escape_writeonly(i32* [[A]])
 ; CHECK-NEXT:    [[IN_LV_2:%.*]] = load i32, i32* [[IN_LV_1]], align 2