Add early-stage optimization crate (#556)

* Add simple constant folding and folding tests

lib/filetests/src/test_simple_preopt.rs

@@ -0,0 +1,44 @@
+//! Test command for testing the preopt pass.
+//!
+//! The resulting function is sent to `filecheck`.
+
+use cranelift_codegen;
+use cranelift_codegen::ir::Function;
+use cranelift_codegen::print_errors::pretty_error;
+use cranelift_reader::TestCommand;
+use std::borrow::Cow;
+use subtest::{run_filecheck, Context, SubTest, SubtestResult};
+
+struct TestSimplePreopt;
+
+pub fn subtest(parsed: &TestCommand) -> SubtestResult<Box<SubTest>> {
+    assert_eq!(parsed.command, "simple_preopt");
+    if !parsed.options.is_empty() {
+        Err(format!("No options allowed on {}", parsed))
+    } else {
+        Ok(Box::new(TestSimplePreopt))
+    }
+}
+
+impl SubTest for TestSimplePreopt {
+    fn name(&self) -> &'static str {
+        "simple_preopt"
+    }
+
+    fn is_mutating(&self) -> bool {
+        true
+    }
+
+    fn run(&self, func: Cow<Function>, context: &Context) -> SubtestResult<()> {
+        let mut comp_ctx = cranelift_codegen::Context::for_function(func.into_owned());
+        let isa = context.isa.expect("preopt needs an ISA");
+
+        comp_ctx.flowgraph();
+        comp_ctx
+            .preopt(isa)
+            .map_err(|e| pretty_error(&comp_ctx.func, context.isa, Into::into(e)))?;
+
+        let text = &comp_ctx.func.display(isa).to_string();
+        run_filecheck(&text, context)
+    }
+}