[Bug tree-optimization/79472] [7 Regression] x86-64: Switch table generation fails if default case has different code

[yuriks at yuriks dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79472

--- Comment #4 from Yuri Kunde Schlesner <yuriks at yuriks dot net> ---
Note that, while this particular example uses a noreturn call, the same pattern
applies to any switch where the default case is the only one which has code
that can't be transformed into a table. For example, replacing `abort();` with
`s = external_getstr();` (`const char* external_getstr();`) follows the same
pattern in all compilers (gcc 6.3 can optimize to table with the nullptr hack,
clang can always optimize, gcc 7 can't optimize at all).

Another thing I noticed is that gcc 6.3 still leaves in the nullptr check after
the switch, even though it's actually unreachable at that point. (Probably out
of scope for this bug though.)