Minor refactors to regalloc

Summary:
Mostly just moving definitions to the .cpp file, keeping the order consistent
between .h and .cpp, and adding comments.

Reviewed By: martindemello

Differential Revision: D81816254

fbshipit-source-id: 4668045a9046daf27273727d273389d33b0b4ee2

Author: alexmalyshev

cinderx/Jit/codegen/gen_asm.cpp

@@ -34,6 +34,7 @@
 #include "cinderx/Jit/lir/generator.h"
 #include "cinderx/Jit/lir/postalloc.h"
 #include "cinderx/Jit/lir/postgen.h"
+#include "cinderx/Jit/lir/printer.h"
 #include "cinderx/Jit/lir/regalloc.h"
 #include "cinderx/Jit/lir/verify.h"
 #include "cinderx/Jit/perf_jitdump.h"