Describe a special case of va_list operand.
* `MIR_VA_ARG` takes va_list and any memory operand and returns
address of the next argument in the 1st insn operand. The memory
operand type defines the type of the argument
+ * va_list operand can be memory with undefined type. In this case
+ address of the va_list is not in the memory but is the
+ memory address
## MIR API example
* The following code on C creates MIR analog of C code
