[MLIR][NFC] Drop unnecessary use of OpBuilder in build trip count map

[bondhugula]

Summary

NFC. Drop unnecessary use of OpBuilder in buildTripCountMapAndOperands.
Rename this to getTripCountMapAndOperands and remove stale comments.

[bondhugula]

Rename method. Remove outdated comments.

[bondhugula]

Add missed update.

[bondhugula]

This is an obvious fix; self accepting and merging.

[bondhugula]

Update commit summary.

mlir/lib/Analysis/LoopAnalysis.cpp

 /// Returns the trip count of the loop as an affine expression if the latter is
 /// expressible as an affine expression, and nullptr otherwise. The trip count
 /// expression is simplified before returning. This method only utilizes map
 /// composition to construct lower and upper bounds before computing the trip
 /// count expressions.
 void mlir::buildTripCountMapAndOperands(
     AffineForOp forOp, AffineMap *tripCountMap,
     SmallVectorImpl<Value> *tripCountOperands) {
-  int64_t loopSpan;
-
+  MLIRContext *context = forOp.getContext();
   int64_t step = forOp.getStep();
-  OpBuilder b(forOp.getOperation());
-
+  int64_t loopSpan;
   if (forOp.hasConstantBounds()) {
     int64_t lb = forOp.getConstantLowerBound();
     int64_t ub = forOp.getConstantUpperBound();
     loopSpan = ub - lb;
     if (loopSpan < 0)
       loopSpan = 0;
-    *tripCountMap = b.getConstantAffineMap(ceilDiv(loopSpan, step));
+    *tripCountMap = AffineMap::getConstantMap(ceilDiv(loopSpan, step), context);
     tripCountOperands->clear();
     return;
   }
   auto lbMap = forOp.getLowerBoundMap();
   auto ubMap = forOp.getUpperBoundMap();
   if (lbMap.getNumResults() != 1) {
     *tripCountMap = AffineMap();
     return;
   }
 
   // Difference of each upper bound expression from the single lower bound
   // expression (divided by the step) provides the expressions for the trip
   // count map.
   AffineValueMap ubValueMap(ubMap, forOp.getUpperBoundOperands());
 
   SmallVector<AffineExpr, 4> lbSplatExpr(ubValueMap.getNumResults(),
                                          lbMap.getResult(0));
   auto lbMapSplat = AffineMap::get(lbMap.getNumDims(), lbMap.getNumSymbols(),
-                                   lbSplatExpr, b.getContext());
+                                   lbSplatExpr, context);
   AffineValueMap lbSplatValueMap(lbMapSplat, forOp.getLowerBoundOperands());
 
   AffineValueMap tripCountValueMap;
   AffineValueMap::difference(ubValueMap, lbSplatValueMap, &tripCountValueMap);
   for (unsigned i = 0, e = tripCountValueMap.getNumResults(); i < e; ++i)
     tripCountValueMap.setResult(i,
                                 tripCountValueMap.getResult(i).ceilDiv(step));