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.

filetests/licm/basic.cton

@@ -0,0 +1,31 @@
+test licm
+
+function simple_loop(i32) -> i32 {
+
+ebb1(v0: i32):
+    v1 = iconst.i32 1
+    v2 = iconst.i32 2
+    v3 = iadd v1, v2
+    brz v0, ebb2(v0)
+    v4 = isub v0, v1
+    jump ebb1(v4)
+
+ebb2(v5: i32):
+    return v5
+
+}
+; sameln: function simple_loop(i32) -> i32 {
+; nextln: ebb2(v6: i32):
+; nextln:     v1 = iconst.i32 1
+; nextln:     v2 = iconst.i32 2
+; nextln:     v3 = iadd v1, v2
+; nextln:     jump ebb0(v6)
+; nextln: 
+; nextln: ebb0(v0: i32):
+; nextln:     brz v0, ebb1(v0)
+; nextln:     v4 = isub v0, v1
+; nextln:     jump ebb0(v4)
+; nextln: 
+; nextln: ebb1(v5: i32):
+; nextln:     return v5
+; nextln: }