Supermicro X8DT6 crashes in bootloader after r239066

I have a X8DT6 that appears to crash in the bootloader from HEAD.  I say
"appears to" because it's difficult to really see what the problem is; the
system reboots pretty much as soon as it enters the FreeBSD boot process.
The problem affects PXE booting, booting from ZFS on GPT on a SATA drive
and UFS on MBR on a USB stick.

The problem only occurs if I have any SATA disks plugged in.  If I remove
all SATA disks I can successfully PXE boot or boot from a USB key.  I've
tried with a couple of different SATA disks, some with GPT and some with
MBR, and the reboot happens in both cases.

The last things that I see on the serial console before the reboot is:

DHCP..-

^[[06;07H^[[06;00HDH
CP..\
^[
[06;07H^[[05;00HCLIENT MAC ADDR: 00 25 90 92 19 94  GUID: 84F7C23B 5F21
2533 625
C 002590921994  ^[[06;00HCLIENT IP: 172.16.1.50  MASK: 255.255.255.0  DHCP
IP: 1
72.16.1.1                ^[[07;00HGATEWAY IP:
172.16.1.1
                                  ^[[08;00HPXE Loader
1.00^[[2JM-^_at_^[[01;00H^[[0
m^[[2m^[[0m^[[2;30;40m
^_at_^[[0m^[[2;37;40m
                                        ^[[02;00H^[[0m^[[2;30;40m

So it really doesn't seem to get very far at all.

I've bisected and confirmed that the problem was introduced in r239066.  I
tried reverting that commit locally and I can boot fine again.  I took a
quick look at that commit but it appears to be way over my head.  I'm
willing to test patches or gather more debugging information; this thing
isn't going anywhere until I can get it booting. :)

Ryan