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/lib.rs

@@ -28,9 +28,11 @@ mod match_directive;
 mod test_binemit;
 mod test_cat;
 mod test_compile;
+mod test_dce;
 mod test_domtree;
 mod test_legalizer;
 mod test_licm;
+mod test_postopt;
 mod test_preopt;
 mod test_print_cfg;
 mod test_regalloc;
@@ -73,9 +75,11 @@ fn new_subtest(parsed: &TestCommand) -> subtest::Result<Box<subtest::SubTest>> {
         "binemit" => test_binemit::subtest(parsed),
         "cat" => test_cat::subtest(parsed),
         "compile" => test_compile::subtest(parsed),
+        "dce" => test_dce::subtest(parsed),
         "domtree" => test_domtree::subtest(parsed),
         "legalizer" => test_legalizer::subtest(parsed),
         "licm" => test_licm::subtest(parsed),
+        "postopt" => test_postopt::subtest(parsed),
         "preopt" => test_preopt::subtest(parsed),
         "print-cfg" => test_print_cfg::subtest(parsed),
         "regalloc" => test_regalloc::subtest(parsed),