enhance(compiler): Use named constant for the default pattern rewriting benefit

This introduces a new header file `zamalang/Support/Constants.h` for
constants, currently only populated with a constant for the default
pattern rewriting benefit of 1.

compiler/include/zamalang/Support/Constants.h

@@ -0,0 +1,10 @@
+#ifndef ZAMALANG_SUPPORT_CONSTANTS_H_
+#define ZAMALANG_SUPPORT_CONSTANTS_H_
+
+namespace mlir {
+namespace zamalang {
+constexpr unsigned DEFAULT_PATTERN_BENEFIT = 1;
+} // namespace zamalang
+} // namespace mlir
+
+#endif