[flang] support fir.alloca operations inside of omp reduction ops

Advise to place the alloca at the start of the first block of whichever
region (init or combiner) we are currently inside.

It probably isn't safe to put an alloca inside of a combiner region
because this will be executed multiple times. But that would be a bug to
fix in Lower/OpenMP.cpp, not here.

Author: tblah

flang/lib/Optimizer/Builder/FIRBuilder.cpp

@@ -208,6 +208,8 @@ mlir::Block *fir::FirOpBuilder::getAllocaBlock() {
               .getParentOfType<mlir::omp::OutlineableOpenMPOpInterface>()) {
     return ompOutlineableIface.getAllocaBlock();
   }
+  if (mlir::isa<mlir::omp::ReductionDeclareOp>(getRegion().getParentOp()))
+    return &getRegion().front();
   if (auto accRecipeIface =
           getRegion().getParentOfType<mlir::acc::RecipeInterface>()) {
     return accRecipeIface.getAllocaBlock(getRegion());

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -410,8 +410,15 @@ class FIROpConversion : public mlir::ConvertOpToLLVMPattern<FromOp> {
       mlir::ConversionPatternRewriter &rewriter) const {
     auto thisPt = rewriter.saveInsertionPoint();
     mlir::Operation *parentOp = rewriter.getInsertionBlock()->getParentOp();
-    mlir::Block *insertBlock = getBlockForAllocaInsert(parentOp);
-    rewriter.setInsertionPointToStart(insertBlock);
+    if (mlir::isa<mlir::omp::ReductionDeclareOp>(parentOp)) {
+      // ReductionDeclareOp has multiple child regions. We want to get the first
+      // block of whichever of those regions we are currently in
+      mlir::Region *parentRegion = rewriter.getInsertionBlock()->getParent();
+      rewriter.setInsertionPointToStart(&parentRegion->front());
+    } else {
+      mlir::Block *insertBlock = getBlockForAllocaInsert(parentOp);
+      rewriter.setInsertionPointToStart(insertBlock);
+    }
     auto size = genI32Constant(loc, rewriter, 1);
     unsigned allocaAs = getAllocaAddressSpace(rewriter);
     unsigned programAs = getProgramAddressSpace(rewriter);