[m-dev.] stage 2 broken with --intermod-opt and -O5

[Julien Fischer]

Bootchecking in asm_fast.gc with --intermod-opt and -O5 is broken
on testing.mercurylang.  Bootchecking on my machine yields the following
during stage 2 compilation

gmake[1]: *** [/tmp/mmake.X8Q1BL:572631:
hlds.make_hlds.make_hlds_passes.c_date] Error 1
gmake[1]: *** Waiting for unfinished jobs....
...
Uncaught Mercury exception:
Software Error: map.lookup: key not found
        Key Type: term.var(parse_tree.prog_data.prog_var_type)
        Key Value: var(12)
        Value Type: ll_backend.var_locn.var_state
Stack dump not available in this grade.

The last ROTD that worked appears to be rotd-2026-07-13.

Julien.