CodeGen: Simplify CodeGenFunction::EmitCaseStmt

Way back in r129652 we tried to avoid emitting an empty block at -O0
for switch cases that did nothing but break. This led to a poor
debugging experience as reported in PR9796, so we disabled the
optimization for -O0 but left it in for higher optimization levels in
r154420.

Since the whole point of this was to improve -O0, it's silly to keep
the complexity at all.