[SEH] Fix assertin when return scalar value from __try block. (#71488)

Current compler assert with `!SI->isAtomic() && !SI->isVolatile()'
failed

This due to following rule:
First, no exception can move in or out of _try region., i.e., no
"potential faulty instruction can be moved across _try boundary. Second,
the order of exceptions for instructions 'directly' under a _try must be
preserved (not applied to those in callees). Finally, global states
(local/global/heap variables) that can be read outside of _try region
must be updated in memory (not just in register) before the subsequent
exception occurs.

All memory instructions inside a _try are considered as 'volatile' to
assure 2nd and 3rd rules for C-code above. This is a little
sub-optimized. But it's acceptable as the amount of code directly under
_try is very small. However during findDominatingStoreToReturnValue:
those are not allowed.

To fix just skip the assertion when current function has seh try.

clang/lib/CodeGen/CGCall.cpp

@@ -3507,7 +3507,9 @@ static llvm::StoreInst *findDominatingStoreToReturnValue(CodeGenFunction &CGF) {
       return nullptr;
     // These aren't actually possible for non-coerced returns, and we
     // only care about non-coerced returns on this code path.
-    assert(!SI->isAtomic() && !SI->isVolatile());
+    // All memory instructions inside __try block are volatile.
+    assert(!SI->isAtomic() &&
+           (!SI->isVolatile() || CGF.currentFunctionUsesSEHTry()));
     return SI;
   };
   // If there are multiple uses of the return-value slot, just check

clang/test/CodeGen/windows-seh-EHa-TryInFinally.cpp

@@ -67,3 +67,26 @@ void foo()
     }
   }
 }
+
+// CHECK-LABEL:@"?bar@@YAHXZ"()
+// CHECK: invoke.cont:
+// CHECK: invoke void @llvm.seh.try.begin()
+// CHECK: invoke.cont1:
+// CHECK: store volatile i32 1, ptr %cleanup.dest.slot
+// CHECK: invoke void @llvm.seh.try.end()
+// CHECK: invoke.cont2:
+// CHECK: call void @"?fin$0@0@bar@@"
+// CHECK: %cleanup.dest3 = load i32, ptr %cleanup.dest.slot
+// CHECK: return:
+// CHECK: ret i32 11
+int bar()
+{
+  int x;
+  __try {
+    return 11;
+  } __finally {
+    if (_abnormal_termination()) {
+      x = 9;
+    }
+  }
+}