cmd/compile: switch to typed aux for mips lowering rules

This covers most of the lowering rules.

Passes

  GOARCH=mips gotip build -toolexec 'toolstash -cmp' -a std
  GOARCH=mipsle gotip build -toolexec 'toolstash -cmp' -a std

Change-Id: I9d00aaebecb36622e3bdaf556e5a9377670bf86b
Reviewed-on: https://go-review.googlesource.com/c/go/+/229102


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