AArch64: Improve codegen in SVE expf & related routines

AArch64: Improve codegen in SVE expf & related routines

Reduce MOV and MOVPRFX by improving special-case handling.  Use inline
helper to duplicate the entire computation between the special- and
non-special case branches, removing the contention for z0 between x
and the return value.

Also rearrange some MLAs and MLSs - by making the multiplicand the
destination we can avoid a MOVPRFX in several cases.  Also change which
constants go in the vector used for lanewise ops - the last lane is no
longer wasted.

Spotted that shift was incorrect in exp2f and exp10f, w.r.t. to the
comment that explains it.  Fixed - worst-case ULP for exp2f moves
around but it doesn't change significantly for either routine.

Worst-case error for coshf increases due to passing x to exp rather
than abs(x) - updated the comment, but does not require regen-ulps.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>