Constant addition to reg-save operands
Fixed lvalue/register-save logic and idiv instruction

When creating stack entries for locals and arguments, we need to set
these to V_REG_SAVE, not V_REG, as they're lvalues on the stack, not the
register+offset themselves. This fixes dereferencing and irons out the
idiv dereference bug.
Dereference and lvalue tests using idiv