JIT: avoid saving the PC on every store

Modify the backpatcher to store the PC in the trampolines.
Should be ~3.5% faster overall (measured on POV-RAY benchmark).
This is a bit weird on my eyes.
It's using a auto it type that was previously used from registersInUseAtLoc.
Sort of messing with my brain bits.
Maybe use a new auto variable with a different name.