The correct way to compare gc.Types is Eqtype,
rather than pointer equality.
Introduce an Equal method for ssa.Type to allow
us to use it.

In the cse pass, use a type's string to build
the coarse partition, and then use Type.Equal
during refinement.

This lets the cse pass do a better job.
In the ~20% of the standard library that SSA
can compile, the number of common subexpressions
recognized by the cse pass increases from
27,550 to 32,199 (+17%). The number of nil checks
eliminated increases from 75 to 115 (+50%).

Change-Id: I0bdbfcf613ca6bc2ec987eb19b6b1217b51f3008
Reviewed-on: https://go-review.googlesource.com/11451Reviewed-by: Keith Randall <khr@golang.org>