[MLIR][OpenMP] Normalize lowering of omp.loop_nest

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())