[flang][StackArrays] run in parallel on different functions (#108842)

Since #108562, StackArrays no longer has to create function declarations
at the module level to use stacksave/stackrestore LLVM intrinsics. This
will allow it to run in parallel on multiple functions at the same time.

Author: tblah

flang/include/flang/Optimizer/Transforms/Passes.td

@@ -278,7 +278,7 @@ def ConstantArgumentGlobalisationOpt : Pass<"constant-argument-globalisation-opt
   let dependentDialects = [ "fir::FIROpsDialect" ];
 }
 
-def StackArrays : Pass<"stack-arrays", "mlir::ModuleOp"> {
+def StackArrays : Pass<"stack-arrays", "mlir::func::FuncOp"> {
   let summary = "Move local array allocations from heap memory into stack memory";
   let description = [{
     Convert heap allocations for arrays, even those of unknown size, into stack

flang/lib/Optimizer/Transforms/StackArrays.cpp

@@ -225,7 +225,6 @@ class StackArraysPass : public fir::impl::StackArraysBase<StackArraysPass> {
   llvm::StringRef getDescription() const override;
 
   void runOnOperation() override;
-  void runOnFunc(mlir::Operation *func);
 
 private:
   Statistic runCount{this, "stackArraysRunCount",
@@ -766,13 +765,7 @@ llvm::StringRef StackArraysPass::getDescription() const {
 }
 
 void StackArraysPass::runOnOperation() {
-  mlir::ModuleOp mod = getOperation();
-
-  mod.walk([this](mlir::func::FuncOp func) { runOnFunc(func); });
-}
-
-void StackArraysPass::runOnFunc(mlir::Operation *func) {
-  assert(mlir::isa<mlir::func::FuncOp>(func));
+  mlir::func::FuncOp func = getOperation();
 
   auto &analysis = getAnalysis<StackArraysAnalysisWrapper>();
   const StackArraysAnalysisWrapper::AllocMemMap *candidateOps =