Previously, we have confused callee-saved with caller-saved registers
(the actual register sets were swapped). This commit fixes that
for both `.x86` and `.x86_64` native backends.
This commit also fixes the register allocation logic in `genBinMathOp`
for `.x86_64` native backend where in a situation such that we require
to spill a register, we would end up spilling the register that is
already involved in the instruction as the other operand. In such a
case, we make a note of this and spill a subsequent register instead.
Jakub Konka