Add constant folding pass (Shopify/zjit#74)

Add a constant folding pass for HIR. This pass uses the types left by
`infer_types` to fold away:

* `GuardType(val, ty)` where `val` is already `ty`
* `FixnumAdd(l, r)` where `l` and `r` are constants
* `FixnumLt(l, r)` where `l` and `r` are constants
* `FixnumEq(l, r)` where `l` and `r` are constants
* `Test(v)`, where `v` is a constant
* `IfTrue(v)` and `IfFalse(v)` where `v` is a constant

It does this by using the union-find data structure in the IR to avoid a
complicated manual find-and-replace process for all the uses. Right now, for
branch instructions that it deletes, we remove from the block completely. This
is safe only because we have ensured that nothing refers to branch
instructions---they produce no values. However, if we want to instead replace
them with Nops that get ignored during codegen or removed by a later DCE pass,
that works for me as well.

Author: tekknolagi

zjit/src/codegen.rs

@@ -80,13 +80,14 @@ fn iseq_gen_entry_point(iseq: IseqPtr) -> *const u8 {
     // with_vm_lock() does nothing if the program doesn't use Ractors.
     with_vm_lock(src_loc!(), || {
         // Compile ISEQ into High-level IR
-        let ssa = match hir::iseq_to_hir(iseq) {
+        let mut ssa = match hir::iseq_to_hir(iseq) {
             Ok(ssa) => ssa,
             Err(err) => {
                 debug!("ZJIT: iseq_to_hir: {:?}", err);
                 return std::ptr::null();
             }
         };
+        ssa.optimize();
 
         // Compile High-level IR into machine code
         let cb = ZJITState::get_code_block();