[mlir][doc] Fix usage of PatternApplicator.

[ingomueller-net]

Summary

The constructor of PatternApplicator doesn't have a constructor that
accepts only a RewritePatternSet as currently used in the example
code in PatternRewriter.md. Instead, one has to turn it into a
FrozenRewritePatternSet.

[jpienaar]

Doesn't FrozenRewritePatternSet have an implicit constructor https://mlir.llvm.org/doxygen/classmlir_1_1FrozenRewritePatternSet.html#ad651d65d1d56ecfdd6290c62f08d5a15 that would apply?

[ingomueller-net]

It seems like this is the inverse direction than what is needed: The constructor you point to constructs a FrozenRewritePatternSet from a RewritePatternSet (which freezes the set); however, in the example, we have a FrozenRewritepatternSet that is given to a function that expected a RewritePatternSet (which would "unfreeze" the set, which I guess shouldn't be possible). My commit makes the function accept a FrozenRewritePatternSet instead, but one could also never freeze it in the first place. I don't really understand the code, so any more informed suggestions are welcome!

mlir/docs/PatternRewriter.md

 public:
   MyPatternRewriter(MLIRContext *ctx) : PatternRewriter(ctx) {}
 
   /// Override the necessary PatternRewriter hooks here.
 };
 
 /// Apply the custom driver to `op`.
 void applyMyPatternDriver(Operation *op,
-                          const RewritePatternSet &patterns) {
+                          const FrozenRewritePatternSet &patterns) {
   // Initialize the custom PatternRewriter.
   MyPatternRewriter rewriter(op->getContext());
 
   // Create the applicator and apply our cost model.
   PatternApplicator applicator(patterns);
   applicator.applyCostModel([](const Pattern &pattern) {
     // Apply a default cost model.
     // Note: This is just for demonstration, if the default cost model is truly