[msan][NFCI] Add test for switch() (#179775)
This test compares the instrumentation for two loosely-equivalent
patterns: (icmp eq + br) vs. switch.
(icmp eq) can have an initialized output even if the inputs are partly
uninitialized, if a bit is initialized in both inputs but has a
different value. In contrast, switch() strictly checks that the input is
fully initialized.
Future work: since the compiler/optimizer may freely choose between
(icmp eq + br) vs. switch, MSan's switch instrumentation also needs to
be able to handle partly-uninitialized inputs.
