[mlir] add permutation utility

[aartbik]

Summary

I found myself typing this code several times at different places
by now, so time to make this a general utility instead. Given
a permutation, it returns the permuted position of the input,
for example (i,j,k) -> (k,i,j) yields position 1 for input 0.

[aartbik]

rebase with main

mlir/include/mlir/IR/AffineMap.h

public:
 
   ArrayRef<AffineExpr> getResults() const;
   AffineExpr getResult(unsigned idx) const;
 
   /// Extracts the position of the dimensional expression at the given result,
   /// when the caller knows it is safe to do so.
   unsigned getDimPosition(unsigned idx) const;
 
+  /// Extracts the permuted position where given input index resides.
+  /// Fails when called on a non-permutation.
+  unsigned getPermutedPosition(unsigned input) const;
+
   /// Return true if any affine expression involves AffineDimExpr `position`.
   bool isFunctionOfDim(unsigned position) const {
     return llvm::any_of(getResults(), [&](AffineExpr e) {
       return e.isFunctionOfDim(position);
     });
   }
 
   /// Return true if any affine expression involves AffineSymbolExpr `position`.

mlir/lib/IR/AffineMap.cpp

AffineExpr AffineMap::getResult(unsigned idx) const {
   assert(map && "uninitialized map storage");
   return map->results[idx];
 }
 
 unsigned AffineMap::getDimPosition(unsigned idx) const {
   return getResult(idx).cast<AffineDimExpr>().getPosition();
 }
 
+unsigned AffineMap::getPermutedPosition(unsigned input) const {
+  assert(isPermutation() && "invalid permutation request");
+  for (unsigned i = 0, numResults = getNumResults(); i < numResults; i++)
+    if (getDimPosition(i) == input)
+      return i;
+  llvm_unreachable("incorrect permutation request");
+}
+
 /// Folds the results of the application of an affine map on the provided
 /// operands to a constant if possible. Returns false if the folding happens,
 /// true otherwise.
 LogicalResult
 AffineMap::constantFold(ArrayRef<Attribute> operandConstants,
                         SmallVectorImpl<Attribute> &results) const {
   // Attempt partial folding.
   SmallVector<int64_t, 2> integers;