V4 JIT: fix stack layout.

Commit d9f33ccd

 introduced an off-by-one
in the calculation of the offset of a saved register (in
StackLayout::savedRegPointer), resulting in overwriting a callee saved
register with the tag of a QV4::Value. This method now calculates those
pointers relative to the bottom of the stack frame.

The off-by-one didn't happen before that patch, because there was a
magical +1 used in the constructor for the number of callee saved
registers, thereby prevented this from happening. However, that resulted
in a frame size that was unnecessary big.

Task-number: QTBUG-40927
Change-Id: If88fe9f3490a4d23a1e69c630c87219fcfef671f
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>