cmd/compile: add -d=ssa/check/seed=SEED

This change adds the option to run the ssa checker with a random seed.
The current system uses a completely fixed seed,
which is good for reproducibility but bad for exploring the state space.

Preserve what we have, but also provide a way for the caller
to provide a seed. The caller can report the seed
alongside any failures.

Change-Id: I2676a8112d8260e6cac86d95d2e8db4d3221aeeb
Reviewed-on: https://go-review.googlesource.com/c/go/+/216418


Reviewed-by: Keith Randall <khr@golang.org>