Restructure cache_handle_range to avoid repeated barriers. Specifically,
restructure cache_handle_range so that all of the data cache operations are
performed before any instruction cache operations. Then, we only need one
barrier between the data and instruction cache operations and one barrier
after the instruction cache operations.

On an Amazon EC2 a1.2xlarge instance, this simple change reduces the time
for a "make -j8 buildworld" by 9%.

Reviewed by: andrew
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20848