V4 JIT: teach regalloc about callee-saved registers.

Callee-saved registers won't get clobbered by a call, so any value
stored in it will survive the call.

Change-Id: I2b61b84617b01bf7ce7e81cd0119a6da689ee9d5
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>