Relax limit on input value for RESCALE

The current value is too strict, and can lead to some cases to fail.
By making the new value limit between (-1 << shift-1) and (1 << shift-1),
we still restrict the input value a range that works for optimized implementations
that left shift their values before multiplying.

Signed-off-by: Eric Kunze <eric.kunze@arm.com>
Change-Id: Ib4f49732e361418884888d398087560da3ccd6e8