Specifying types in rewrites for all subexpressions gets verbose
quickly.  Allow opcodes to specify a default type which is used when
none is supplied explicitly.

Provide default types for a few easy opcodes.  There are probably more
we can do, but this is a good start.

Change-Id: Iedc2a1a423cc3e2d4472640433982f9aa76a9f18
Reviewed-on: https://go-review.googlesource.com/14128Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>