[MLIR][OpenMP] Normalize lowering of omp.loop_nest (#127217)

This patch refactors the translation of `omp.loop_nest` operations into
LLVM IR so that it is handled similarly to other operations. Before this
change, the responsibility of translating the loop nest fell into each
loop wrapper, causing code duplication. This patch centralizes that
handling of the loop. One consequence of this was fixing an issue
lowering non-inclusive `omp.simd` loops.

As a result, it is now expected that the handling of composite
constructs is performed collaboratively among translating functions for
each operation involved. At the moment, only `do/for simd` is supported
by ignoring SIMD information, and this behavior is preserved.

The translation of loop wrapper operations needs access to the
`llvm::CanonicalLoopInfo` loop information structure in order to apply
transformations to it. This is now created in the nested call to
`convertOmpLoopNest`, so it needs to be passed up to all associated loop
wrapper translation functions. This is done via the creation of an
`OpenMPLoopInfoStackFrame` within `convertHostOrTargetOperation`,
associated to the outermost loop wrapper. This structure is updated by
`convertOmpLoopNest`, making the result available to all loop wrappers
after their body has been translated.

Author: skatrak

mlir/include/mlir/Target/LLVMIR/ModuleTranslation.h

@@ -290,13 +290,12 @@ class ModuleTranslation {
   /// Calls `callback` for every ModuleTranslation stack frame of type `T`
   /// starting from the top of the stack.
   template <typename T>
-  WalkResult
-  stackWalk(llvm::function_ref<WalkResult(const T &)> callback) const {
+  WalkResult stackWalk(llvm::function_ref<WalkResult(T &)> callback) {
     static_assert(std::is_base_of<StackFrame, T>::value,
                   "expected T derived from StackFrame");
     if (!callback)
       return WalkResult::skip();
-    for (const std::unique_ptr<StackFrame> &frame : llvm::reverse(stack)) {
+    for (std::unique_ptr<StackFrame> &frame : llvm::reverse(stack)) {
       if (T *ptr = dyn_cast_or_null<T>(frame.get())) {
         WalkResult result = callback(*ptr);
         if (result.wasInterrupted())