Fix remaining AMP fallout.

The perf tests can probably be rechecked and tightened a little; I fixed
them with AMP the other day but some changes since then have made them
wibble perhaps.

Signed-off-by: Austin Seipp <austin@well-typed.com>
Test for #8205

This test is a bit speculative, because I can't reproduce problem
on my machine. Still, it should work because it produces the same
Cmm that originally caused the problem.