Refactor compiler so that the low level code generation functions operate on a basicblock*

[iritkatriel]

The compiler's functions that add instructions (compiler_addop, etc) take a struct compiler * arg, so they cannot be reused by the optimiser/assembler. They should be refactored so that they only take a basicblock*.

While I'm at it, I will roll compiler_check_if_end_of_block into the subsequent compiler_use_new_implicit_block_if_needed (which can look at the last instruction in the block).