[dev.ssa] cmd/compile/internal/ssa: Add code to test generated opcode counts

[dev.ssa] cmd/compile/internal/ssa: Add code to test generated opcode counts

Add test handler to count and check generated opcodes.  This will be
useful for testing that certain optimizations don't regress.

Also pass a *Config to the Fun constructor so that compile() works.

Change-Id: Iee679e87cf0bc635ddcbe433fc1bd4c1d9c953cc
Reviewed-on: https://go-review.googlesource.com/10502
Reviewed-by: Michael Matloob <michaelmatloob@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>