cmd/compile/internal/ssa: recognize constant pointer comparison

cmd/compile/internal/ssa: recognize constant pointer comparison

Teach the backend to recognize that the address of a symbol
is equal with itself, and that the addresses of two different
symbols are different.

Some examples of where this rule hits in the standard library:

 - inlined uses of (*time.Time).setLoc (e.g. time.UTC)
 - inlined uses of bufio.NewReader (via type assertion)

Change-Id: I23dcb068c2ec333655c1292917bec13bbd908c24
Reviewed-on: https://go-review.googlesource.com/38338
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>