cmd/compile: add MIPS64 optimizations, SSA on by default

cmd/compile: add MIPS64 optimizations, SSA on by default

Add the following optimizations:
- fold constants
- fold address into load/store
- simplify extensions and conditional branches
- remove nil checks

Turn on SSA on MIPS64 by default, and toggle the tests.

Fixes #16359.

Change-Id: I7f1e38c2509e22e42cd024e712990ebbe47176bd
Reviewed-on: https://go-review.googlesource.com/27870
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>