NetBSD compatibility report for SGI O2 (ip3x) - NetBSD 8.2 works, 9.0 and 9.1 don't seem to

Hi folks,

I have an old SGI O2 (180MHz R5K) that I'd not turned on in about a
decade. I fired it up, and it had NetBSD 5.0 on it, so I decided to do
an upgrade. I booted the NetBSD 9.1 ISO from my SCSI2SD v5.2 (using
CD-ROM emulation), and ARCS loads the bootstrap, but the process fails
seven lines of output in, with:

"adding 0xec00000 @ 0x1400000 to freelist 0"

I then tried with NetBSD 9.0, the kernel loads, and video output gets
about 25 lines of output, up to the point where it initializes crmfb0,
and then there's no further video output, and the machine appears
frozen. I haven't tried to capture serial output, because I'm not sure
if NetBSD on sgimips emits any, without configuration.

In then tried with NetBSD 8.2 which boots without issue. So, somewhere
in there is a regression, and while I don't have the skills to figure
out when the breakage happened, I figure it would at least be
appropriate to document it for the sake of others.