This problem is the cause of Mantis bug #452.
compiler/switch_util.m:
Don't require a bit vector check for cannot_fail switches,
even in cases where the switch arms do not completely cover
the range of possible values in the switched-on variable's type;
if the switch is cannot_fail, then the switch variable cannot have
any of the "missing" values when the switch is entered.
tests/hard_coded/bug452.{m,exp}:
A regression test for the bug.
tests/hard_coded/Mmakefile:
Enable the new test case.
