Change to table-driven instead of branchy code; leads to
net reduction in lines, easier to understand what happens,
easier to modify code if we want option to exclude generation
of branchy cases.

Doesn't appear to scale for 8x8 case of integer types.

Change-Id: Ib40104b149d30bb329c5782f6cac45c75743e768
Reviewed-on: https://go-review.googlesource.com/14163Reviewed-by: Keith Randall <khr@golang.org>