By moving exported methods to the front of method lists, filtering
down to only the exported methods just needs a count of how many
exported methods exist, which the compiler can statically
provide. This allows getting rid of the exported method cache.

For #22075.

Change-Id: I8eeb274563a2940e1347c34d673f843ae2569064
Reviewed-on: https://go-review.googlesource.com/100846Reviewed-by: Ian Lance Taylor <iant@golang.org>