Fix several 'reaches end of function' warnings by making fatal noreturn.

The warnings also pointed out a few genuine bugs. Replace some tabs with
spaces.

gen/irstate.cpp

@@ -39,6 +39,7 @@ const IRScope& IRScope::operator=(const IRScope& rhs)
     begin = rhs.begin;
     end = rhs.end;
     builder.SetInsertPoint(begin);
+    return *this;
 }
 
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