[test] skip invalid -O2 test [GH #1049]

t/compilers/imcc/syn/regressions.t contains testing for a run-time exception
which is optimized away by constant propagation and then used_once
at compile-time with -O2.

$ ./parrot -d1040 -O2 t/compilers/imcc/syn/regressions_2.pir
used once deleted 0x2289920 div $I1, 1, 0
used once deleted 0x228a0f0 div $N1, 1.0, 0.0
code_size(ops) 17  old_size 0
0 push_eh_ic	0x251d6c0 push_eh ok1
2 pop_eh	0x251d9d0 pop_eh
3 print_sc 0	0x251db30 print "not "
5 say_sc 2	0x251dd30 say "ok 1 - caught div_i_ic_ic exception"
7 push_eh_ic	0x251de90 push_eh ok2
9 pop_eh	0x251e1a0 pop_eh
10 print_sc 0	0x251e260 print "not "
12 say_sc 1	0x251e430 say "ok 2 - caught div_n_nc_nc exception"
14 set_returns_pc 0	0x251ed60 set_returns
16 returncc	0x251ede0 returncc