Introducing the dead code elimination phase

[takagi]

The dead code elimination phase is to be introduced within the compiler phases for optimization.

[takagi]

For now, dead code elimination is introduced on 5e41949 and b7dde69.

This eliminates the following kinds of dead codes:

• local function definitions
• variable definitions
• top-level function definitions

Currently, top-level functions' dead code elimination is imcomplete because indirect dead codes are not eliminated. For example, foo, called by bar that is not called anywhere, is not eliminated.

foo -> bar -> x

Currently the following forms are supposed to be with side effects:

• SETREG forms
• function applications (because no ways to know if being with side effects)

[takagi]

Now, on ad6f4e5, indirectly dead codes in top-level function definitions are eliminated.