Fix findAccessedStorage to handle cyclic phis.
Fixes infinite recursion in findAccessedStorge. This routine was not
designed to be recursive, but recursion was recently added as a hack
for address-type phis. Naturally, phis can be cyclic, so this was not
a correct workaround.
This is only a problem with enforce-exclusivity=checked, with which
findAccessedStorge is invoked after arbitrary optimization passes that
introduce address-type phis. Ultimately, address phis will be
disallowed in SIL, but some optimizer passes must be fixed first.
Fixes <rdar://problem/47059671> swiftlang-1001.0.31.11 root: swiftc crashes
