Optimize immediates and compare and branch sequences (#286)

* Add a pre-opt optimization to change constants into immediates.

This converts 'iadd' + 'iconst' into 'iadd_imm', and so on.

* Optimize away redundant `bint` instructions.

Cretonne has a concept of "Testable" values, which can be either boolean
or integer. When the an instruction needing a "Testable" value receives
the result of a `bint`, converting boolean to integer, eliminate the
`bint`, as it's redundant.

* Postopt: Optimize using CPU flags.

This introduces a post-legalization optimization pass which converts
compare+branch sequences to use flags values on CPUs which support it.

* Define a form of x86's `urm` that doesn't clobber FLAGS.

movzbl/movsbl/etc. don't clobber FLAGS; define a form of the `urm`
recipe that represents this.

* Implement a DCE pass.

This pass deletes instructions with no side effects and no results that
are used.

* Clarify ambiguity about "32-bit" and "64-bit" in comments.

* Add x86 encodings for icmp_imm.

* Add a testcase for postopt CPU flags optimization.

This covers the basic functionality of transforming compare+branch
sequences to use CPU flags.

* Pattern-match irsub_imm in preopt.

lib/filetests/src/test_postopt.rs

@@ -0,0 +1,50 @@
+//! Test command for testing the postopt pass.
+//!
+//! The resulting function is sent to `filecheck`.
+
+use cretonne::ir::Function;
+use cretonne;
+use cretonne::print_errors::pretty_error;
+use cton_reader::TestCommand;
+use subtest::{SubTest, Context, Result, run_filecheck};
+use std::borrow::Cow;
+use std::fmt::Write;
+
+struct TestPostopt;
+
+pub fn subtest(parsed: &TestCommand) -> Result<Box<SubTest>> {
+    assert_eq!(parsed.command, "postopt");
+    if !parsed.options.is_empty() {
+        Err(format!("No options allowed on {}", parsed))
+    } else {
+        Ok(Box::new(TestPostopt))
+    }
+}
+
+impl SubTest for TestPostopt {
+    fn name(&self) -> Cow<str> {
+        Cow::from("postopt")
+    }
+
+    fn is_mutating(&self) -> bool {
+        true
+    }
+
+    fn run(&self, func: Cow<Function>, context: &Context) -> Result<()> {
+        // Create a compilation context, and drop in the function.
+        let mut comp_ctx = cretonne::Context::new();
+        comp_ctx.func = func.into_owned();
+        let isa = context.isa.expect("postopt needs an ISA");
+
+        comp_ctx.flowgraph();
+        comp_ctx.postopt(isa).map_err(|e| {
+            pretty_error(&comp_ctx.func, context.isa, Into::into(e))
+        })?;
+
+        let mut text = String::new();
+        write!(&mut text, "{}", &comp_ctx.func).map_err(
+            |e| e.to_string(),
+        )?;
+        run_filecheck(&text, context)
+    }
+}