[flang][OpenACC] simplify copy and combiner recipe generation (#164988)

OpenACC copy and combiner recipe generation had non-trivial
ad-hoc handling of the FIR types, and ended-up generating hlfir.assign
that become runtime calls for arrays because of the lack of aliasing
guarantees given to FIR for the region arguments.

This code was started before HFLIR, and with HLFIR tools, most of the
type specific handling can just be removed to use hlfir generic helper
and assign (-230 lines in OpenACC.cpp).

To avoid ending up with runtime calls in recipes:
- use hlfir.assign temporary_lhs in copies to indicate that the copy
cannot alias with the rhs.
- add a new genNoAliasAssignment hlfir helper that takes its logic from
the HLFIRInlineIntrinsic pass and allows applying a scalar combiner
before generating the scalar assignments. This allows generating loops
directly for the reduction and avoid having to find a clever way to
signal HLFIR that the arguments do not alias (an other option would have
been to introduce a dummy_scope and declares, but that would not be
enough for POINTERs, and I am not sure we should start using this as a
noalias operation).

Author: jeanPerier

flang/include/flang/Optimizer/Builder/HLFIRTools.h

@@ -450,6 +450,41 @@ mlir::Value inlineElementalOp(
     mlir::IRMapping &mapper,
     const std::function<bool(hlfir::ElementalOp)> &mustRecursivelyInline);
 
+/// Generate an element-by-element assignment from \p rhs to \p lhs for arrays
+/// that are known not to alias. The assignment is performed using a loop nest
+/// over the optimal extents deduced from both shapes. If \p emitWorkshareLoop
+/// is true, a workshare loop construct may be emitted when available.
+/// Allocatable LHS must be allocated with the right shape and parameters.
+void genNoAliasArrayAssignment(
+    mlir::Location loc, fir::FirOpBuilder &builder, hlfir::Entity rhs,
+    hlfir::Entity lhs, bool emitWorkshareLoop = false,
+    bool temporaryLHS = false,
+    std::function<hlfir::Entity(mlir::Location, fir::FirOpBuilder &,
+                                hlfir::Entity, hlfir::Entity)> *combiner =
+        nullptr);
+
+/// Generate an assignment from \p rhs to \p lhs when they are known not to
+/// alias. Handles both arrays and scalars: for arrays, delegates to
+/// genNoAliasArrayAssignment; for scalars, performs load/store for trivial
+/// scalar types and falls back to hlfir.assign otherwise.
+/// Allocatable LHS must be allocated with the right shape and parameters.
+void genNoAliasAssignment(
+    mlir::Location loc, fir::FirOpBuilder &builder, hlfir::Entity rhs,
+    hlfir::Entity lhs, bool emitWorkshareLoop = false,
+    bool temporaryLHS = false,
+    std::function<hlfir::Entity(mlir::Location, fir::FirOpBuilder &,
+                                hlfir::Entity, hlfir::Entity)> *combiner =
+        nullptr);
+inline void genNoAliasAssignment(
+    mlir::Location loc, fir::FirOpBuilder &builder, hlfir::Entity rhs,
+    hlfir::Entity lhs, bool emitWorkshareLoop, bool temporaryLHS,
+    std::function<hlfir::Entity(mlir::Location, fir::FirOpBuilder &,
+                                hlfir::Entity, hlfir::Entity)>
+        combiner) {
+  genNoAliasAssignment(loc, builder, rhs, lhs, emitWorkshareLoop, temporaryLHS,
+                       &combiner);
+}
+
 /// Create a new temporary with the shape and parameters of the provided
 /// hlfir.eval_in_mem operation and clone the body of the hlfir.eval_in_mem
 /// operating on this new temporary.  returns the temporary and whether the