r600/sb: cleanup if_conversion iterator to be legal C++

The current code causes:
/usr/include/c++/8/debug/safe_iterator.h:207:
Error: attempt to copy from a singular iterator.

This is due to the iterators getting invalidated, fix the
reverse iterator to use the return value from erase, and
cast it properly.

(used Mathias suggestion)
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Mathias Fröhlich <mathias.froehlich@web.de>

(cherry picked from commit 8c51caab)