[mlir][scf] Implement getSingle... of LoopLikeOpInterface for scf::ForallOp (#67883)

The `getSingle(IterationVar|UpperBound|LowerBound|Step)` methods of
`LoopLikeOpInterface` are useful to quickly query the iteration space of 
unidimensional loops. Until now, `scf::ForallOp` always fell back to the
default implementation of these methods, returning `std::nullopt`.

This patch implements those methods, returning the respective bounds
or steps in the special case of `rank == 1`.

Author: ubfx

mlir/include/mlir/Dialect/SCF/IR/SCFOps.td

@@ -333,7 +333,8 @@ def ForallOp : SCF_Op<"forall", [
        AttrSizedOperandSegments,
        AutomaticAllocationScope,
        DeclareOpInterfaceMethods<LoopLikeOpInterface,
-          ["promoteIfSingleIteration"]>,
+          ["promoteIfSingleIteration", "getSingleInductionVar",
+          "getSingleLowerBound", "getSingleUpperBound", "getSingleStep"]>,
        RecursiveMemoryEffects,
        SingleBlockImplicitTerminator<"scf::InParallelOp">,
        DeclareOpInterfaceMethods<RegionBranchOpInterface>,

mlir/lib/Dialect/SCF/IR/SCF.cpp

@@ -1527,6 +1527,30 @@ InParallelOp ForallOp::getTerminator() {
   return cast<InParallelOp>(getBody()->getTerminator());
 }
 
+std::optional<Value> ForallOp::getSingleInductionVar() {
+  if (getRank() != 1)
+    return std::nullopt;
+  return getInductionVar(0);
+}
+
+std::optional<OpFoldResult> ForallOp::getSingleLowerBound() {
+  if (getRank() != 1)
+    return std::nullopt;
+  return getMixedLowerBound()[0];
+}
+
+std::optional<OpFoldResult> ForallOp::getSingleUpperBound() {
+  if (getRank() != 1)
+    return std::nullopt;
+  return getMixedUpperBound()[0];
+}
+
+std::optional<OpFoldResult> ForallOp::getSingleStep() {
+  if (getRank() != 1)
+    return std::nullopt;
+  return getMixedStep()[0];
+}
+
 ForallOp mlir::scf::getForallOpThreadIndexOwner(Value val) {
   auto tidxArg = llvm::dyn_cast<BlockArgument>(val);
   if (!tidxArg)