repro:

# was only able to repro with this combination of one
# case, one pass and a repeated pass. Order doesn't seem to matter.
switch 10
pass 3
pass 3
do
    ;
case 1
    ;
pass 2
do
    ;
default
    ;
#
    terminate called after throwing an instance of 'std::bad_alloc'
    what():  std::bad_alloc
    Stack dump:
    0.      Running pass 'Function Pass Manager' on module '/home/[redacted]/backend-abort-repro.sc:3'.
    1.      Running pass 'X86 DAG->DAG Instruction Selection' on function '@"/home/[redacted]/backend-abort-repro.sc:3<>$f1f7646c81ab64326_2"'