stack pointer before calling, unless we pushed arguments by stack)

a function

cannot run test_throw in test_rpython along with other pytests
(especially tests from c script). Will look into this later

1. grow/shrink frame size in the pro/epilogue. Note: though we removed
push/pop for unused callee saved registers, we still reserve frame space
for them. Because we generate spill on frame before we know exactly how
large a space is. The solution is to make spill locations patchable.
2. control flow will combine branches in switch instruction if they all
target the same destination.

(for callee saved regs, we insert push/pop before reg alloc. and
after allocation, remove unnecessary ones)

internal id and machine id do not persist during serialization
and restoration. Currently I use fixed ID for machine registers.
But internal id is still generated, this might be a problem in
the future.

partially serialize compiled function into bootimage