Utilize Binaryen's -O4

When specifying -O3 or -Oz, asc now automatically increases the optimize level to 4, then including costly non-LLVM optimizations for blocky code, in turn achieving similar results as an LLVM-based generator.

tests/compiler/class-with-boolean-field.optimized.wat

@@ -4,13 +4,12 @@
  (export "test" (func $class-with-boolean-field/test))
  (export "memory" (memory $0))
  (func $class-with-boolean-field/test (; 0 ;) (type $i) (result i32)
-  (local $0 i32)
   (i32.store8
-   (get_local $0)
+   (i32.const 0)
    (i32.const 1)
   )
   (i32.load8_u
-   (get_local $0)
+   (i32.const 0)
   )
  )
 )