Directly store constants as immediate Value's

Directly store constants as immediate Value's

This removes a link from the JIT code to the IR,
and should improve performance as we need slightly
less copies.
On 64 bit, Values are stores in 2 instructions instead
of 4 now.

Change-Id: I4a90165fcdfc2d77ac3f6e7e9accdef8600fadd6
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>