[patch] Make every GIMPLE switch have a default case, always

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

Hello,

This patch restores the old invariant that every GIMPLE switch has a
default case. This invariant is only broken by the SJLJ exception
dispatch code, and it's resulted in some code accepting a switch
without a default while others still assume there is _always_ a
default case. The patch enforces the invariant, fixes some fall-out,
and cleans up the code in a couple of places. It makes the follow-up
work on switch code generation that I still have planned a bit easier.

Bootstrapped&tested on x86_64-unknown-linux-gnu (with Java to torture
the exception handling code) and did a non-bootstrap build (including
Java again) with --enable-sjlj-exceptions. OK for trunk?

Ciao!
Steven

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