Fix the wrong QMax and QMin type assignment in rescale op

Signed integer type is used to retain QMax and QMin no matter what
the value of `output_unsigned` is, but the value of QMax and QMin
are unsigned integer when output_unsigned is true.

Also add a handful of arithmetic helpers to align the pseudo code.

Change-Id: Ie3cd444a290ddae08884186cd4b349a88acad032
Signed-off-by: TatWai Chong <tatwai.chong@arm.com>