[NFC][wasm-reduce] Split out DestructiveReducer

The Reducer class used to do double duty providing general reducer
utilities and also walking the module to do destructive reduction. Split
the module walking part into a new DestructiveReducer class so the
Reducer class providing general utilities can be reasonably reused by
new reducer classes in the future.

Author: tlively

src/tools/wasm-reduce/CMakeLists.txt

@@ -1,7 +1,8 @@
 FILE(GLOB wasm_reduce_HEADERS *h)
 set(wasm_reduce_SOURCES
-  wasm-reduce.cpp
+  destructive-reducer.cpp
   reducer.cpp
+  wasm-reduce.cpp
   ${wasm_reduce_HEADERS}
 )
 binaryen_add_executable(wasm-reduce "${wasm_reduce_SOURCES}")