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.

cranelift/docs/testing.rst

@@ -334,6 +334,14 @@ Test the LICM pass.
 The LICM pass is run on each function, and then results are run
 through filecheck.
 
+`test dce`
+-----------------
+
+Test the DCE pass.
+
+The DCE pass is run on each function, and then results are run
+through filecheck.
+
 `test preopt`
 -----------------
 
@@ -342,6 +350,14 @@ Test the preopt pass.
 The preopt pass is run on each function, and then results are run
 through filecheck.
 
+`test postopt`
+-----------------
+
+Test the postopt pass.
+
+The postopt pass is run on each function, and then results are run
+through filecheck.
+
 `test compile`
 --------------