`(a*)*` spins the matching engine forever

[hamishknight]

Attempting to match the input "a" against the pattern (a*)* appears to spin the matching engine forever.

rdar://96461197

[hamishknight]

cc @milseman

[milseman]

This is a common/natural fall out of simple backtracking. We'll want to eventually have loop analysis over an IR to avoid it, but there might be a shorter term fix we can do.