* Closes#2829
* Adds a transformation which converts `br` to `if` when the variable
branched on was assigned in the previous instruction. The transformation
itself doesn't check liveness and doesn't remove the assignment. Dead
code elimination should be run afterwards to remove the assignment.
* For Cairo, it only makes sense to convert `br` to `if` for equality
comparisons against zero. The assignment before `br` will always become
dead after converting `br` to `if`, because we convert to SSA before.
After this change there are only two types of variables in JuvixReg:
arguments and local variables. The previous distinction between stack
and temporary variables was spurious and complicated things
unnecessarily.
