[flang][NFC] use hlfir.declare first result when both results are raw pointers (#132261)

Currently, the helpers to get fir::ExtendedValue out of hlfir::Entity
use hlfir.declare second result (`#1`) in most cases. This is because
this result is the same as the input and matches what FIR was getting
before lowering to HLFIR.

But this creates odd situations when both hlfir.declare are raw pointers
and either result ends-up being used in the IR depending on whether the
code was generated by a helper using fir::ExtendedValue, or via "pure
HLFIR" helpers using the first result.

This will typically prevent simple CSE and easy identification that two
operation (e.g load/store) are touching the exact same memory location
without using alias analysis or "manual detection" (looking for common
hlfir.declare defining op).

Hence, when hlfir.declare results are both raw pointers, use `#0` when
producing `fir::ExtendedValue`.
When `#0` is a fir.box, keep using `#1` because these are not the same. 
The only code change is in HLFIRTools.cpp and is pretty small, but there
is a big test fallout of `#1` to `#0`.

Author: jeanPerier

Diff for: flang/lib/Optimizer/Builder/HLFIRTools.cpp

@@ -186,14 +186,24 @@ static fir::CharBoxValue genUnboxChar(mlir::Location loc,
   return {addr, len};
 }
 
+// To maximize chances of identifying usage of a same variables in the IR,
+// always return the hlfirBase result of declare/associate if it is a raw
+// pointer.
+static mlir::Value getFirBaseHelper(mlir::Value hlfirBase,
+                                    mlir::Value firBase) {
+  if (fir::isa_ref_type(hlfirBase.getType()))
+    return hlfirBase;
+  return firBase;
+}
+
 mlir::Value hlfir::Entity::getFirBase() const {
   if (fir::FortranVariableOpInterface variable = getIfVariableInterface()) {
     if (auto declareOp =
             mlir::dyn_cast<hlfir::DeclareOp>(variable.getOperation()))
-      return declareOp.getOriginalBase();
+      return getFirBaseHelper(declareOp.getBase(), declareOp.getOriginalBase());
     if (auto associateOp =
             mlir::dyn_cast<hlfir::AssociateOp>(variable.getOperation()))
-      return associateOp.getFirBase();
+      return getFirBaseHelper(associateOp.getBase(), associateOp.getFirBase());
   }
   return getBase();
 }

Diff for: flang/test/HLFIR/all-lowering.fir

@@ -90,7 +90,7 @@ func.func @_QPall3(%arg0: !fir.ref<!fir.array<2x!fir.logical<4>>> {fir.bindc_nam
 
 // CHECK-DAG:     %[[MASK_ADDR:.*]] = fir.address_of
 // CHECK-DAG:     %[[MASK_VAR:.*]]:2 = hlfir.declare %[[MASK_ADDR]](%[[MASK_SHAPE:.*]])
-// CHECK-DAG:     %[[MASK_BOX:.*]] = fir.embox %[[MASK_VAR]]#1(%[[MASK_SHAPE:.*]])
+// CHECK-DAG:     %[[MASK_BOX:.*]] = fir.embox %[[MASK_VAR]]#0(%[[MASK_SHAPE:.*]])
 
 // CHECK-DAG:     %[[DIM:.*]] = arith.constant 1 : i32