Optimise load and bit manipulation instructions applied to constant structures

[pschachte]

For example, Wybe string constants always have the cstring constructor, so an instruction that masks and compares a string constant pointer should know what the tag will be. The following instruction that loads the cstring pointer can also know what the value will be.

This can also work when an instruction is applied to a constant construction, such as [1,2,3]. In that case, there will be a sequence of instructions to create that structure, but the tags and values in that structure are known ahead, so we can know what bit manipulation and load instructions will do to parts of that structure. That even applies to partially constant structures, like [1,2,x].