[MLIR] Expose approximation patterns for tanh/erf. (#82750)

These patterns can already be used via
populateMathPolynomialApproximationPatterns, but that includes a number
of other patterns that may not be needed.

There are already similar functions for expansion.

For now only adding tanh and erf since I have a concrete use case for
these two.

Author: jreiffers

mlir/include/mlir/Dialect/Math/Transforms/Passes.h

@@ -45,6 +45,9 @@ struct MathPolynomialApproximationOptions {
   bool enableAvx2 = false;
 };
 
+void populatePolynomialApproximateTanhPattern(RewritePatternSet &patterns);
+void populatePolynomialApproximateErfPattern(RewritePatternSet &patterns);
+
 void populateMathPolynomialApproximationPatterns(
     RewritePatternSet &patterns,
     const MathPolynomialApproximationOptions &options = {});

mlir/lib/Dialect/Math/Transforms/PolynomialApproximation.cpp

@@ -1471,6 +1471,16 @@ RsqrtApproximation::matchAndRewrite(math::RsqrtOp op,
 
 //----------------------------------------------------------------------------//
 
+void mlir::populatePolynomialApproximateTanhPattern(
+    RewritePatternSet &patterns) {
+  patterns.add<TanhApproximation>(patterns.getContext());
+}
+
+void mlir::populatePolynomialApproximateErfPattern(
+    RewritePatternSet &patterns) {
+  patterns.add<ErfPolynomialApproximation>(patterns.getContext());
+}
+
 void mlir::populateMathPolynomialApproximationPatterns(
     RewritePatternSet &patterns,
     const MathPolynomialApproximationOptions &options) {