V4 JIT: add IR printer that uses platform register names.

After running register allocation, the IR is printed one last time (when
the environment variable QV4_SHOW_IR is set). This will now use the
platform defined register names, e.g. "ebx" or "d2".

Change-Id: I2f2f4536d1de940a69690056e5165d38fa7910bb
Reviewed-by: Lars Knoll <lars.knoll@digia.com>