Constant propagation in word_simp

[tanyongkiam]

At the moment, constant foldng/propagation in wordLang is limited to expressions:

https://github.com/CakeML/cakeml/blob/master/compiler/backend/word_simpScript.sml#L156

This issue is about extending it to work for instructions as well:

https://github.com/CakeML/cakeml/blob/master/compiler/backend/word_simpScript.sml#L220

(and probably other simple places that could benefit).

This was reported by @talsewell in the context of Pancake programs.

[talsewell]

For the purposes of pancake programs, this might be fixed by #1169 , assuming that PR succeeds.