Handle CAST from fp-type to narrow integer types

Be explicit that the value is converted to int32 and then clamped to the
output type range.

Also remove use of apply_clip, replacing with either apply_clip_s or apply_clip_u

Signed-off-by: Eric Kunze <eric.kunze@arm.com>
Change-Id: I0efc7f4d653c1bef13c9989cfe2647208dad9617