LICM pass (#87)

* LICM pass

* Uses loop analysis to detect loop tree
* For each loop (starting with the inner ones), create a pre-header and move there loop-invariant instructions
* An instruction is loop invariant if it does not use as argument a value defined earlier in the loop
* File tests to check LICM's correctness
* Optimized pre-header creation
If the loop already has a natural pre-header, we use it instead of creating a new one.
The natural pre-header of a loop is the only predecessor of the header it doesn't dominate.

cranelift/src/filetest/mod.rs

@@ -17,6 +17,7 @@ mod binemit;
 mod concurrent;
 mod domtree;
 mod legalizer;
+mod licm;
 mod regalloc;
 mod runner;
 mod runone;
@@ -61,6 +62,7 @@ fn new_subtest(parsed: &TestCommand) -> subtest::Result<Box<subtest::SubTest>> {
         "domtree" => domtree::subtest(parsed),
         "verifier" => verifier::subtest(parsed),
         "legalizer" => legalizer::subtest(parsed),
+        "licm" => licm::subtest(parsed),
         "regalloc" => regalloc::subtest(parsed),
         "binemit" => binemit::subtest(parsed),
         "simple-gvn" => simple_gvn::subtest(parsed),