[Flang][OpenMP] Prevent allocas from being inserted into loop wrappers

This patch updates the `FirOpBuilder::getAllocaBlock()` method to avoid
returning blocks which are part of a region owned by a loop wrapper operation.

This avoids introducing `fir.alloca` operations inside of loop wrappers, which
would cause verifier errors due to strict restrictions on their allowed
contents. These allocations will be created in the entry block of the first
non-loop wrapper parent region.

Author: skatrak

flang/lib/Optimizer/Builder/FIRBuilder.cpp

@@ -259,6 +259,29 @@ mlir::Block *fir::FirOpBuilder::getAllocaBlock() {
     return recipeIface.getAllocaBlock(getRegion());
   }
 
+  // All allocations associated with an OpenMP loop wrapper must happen outside
+  // of all wrappers.
+  mlir::Operation *currentOp = getRegion().getParentOp();
+  auto wrapperIface =
+      llvm::isa<mlir::omp::LoopNestOp>(currentOp)
+          ? llvm::cast<mlir::omp::LoopWrapperInterface>(
+                currentOp->getParentOp())
+          : llvm::dyn_cast<mlir::omp::LoopWrapperInterface>(currentOp);
+  if (wrapperIface) {
+    // Cannot use LoopWrapperInterface methods here because the whole nest may
+    // not have been created at this point. Manually traverse parents instead.
+    mlir::omp::LoopWrapperInterface lastWrapperOp = wrapperIface;
+    while (true) {
+      if (auto nextWrapper =
+              llvm::dyn_cast_if_present<mlir::omp::LoopWrapperInterface>(
+                  lastWrapperOp->getParentOp()))
+        lastWrapperOp = nextWrapper;
+      else
+        break;
+    }
+    return &lastWrapperOp->getParentRegion()->front();
+  }
+
   return getEntryBlock();
 }