dwarf2out.cc (mem_loc_descriptor): Handle BITREVERSE

dwarf2out.cc (mem_loc_descriptor): Handle BITREVERSE

This seems to have just been overlooked when introducing
BITREVERSE.  Note that the function name mem_loc_descriptor
is a misnomer; it'd better be called rtx_loc_descriptor or
any_loc_descriptor, because "anything" RTX can end up here.
To wit, when introducing new RTL that ends up as code or for
other reasons appear in debug expressions, don't forget to
update this function.  This was observed by building
libstdc+++ for cris-elf with a patch replacing the
CRIS_UNSPEC_SWAP_BITS by bitreverse, as hitting the
internal-error-generating default case.

Looking at the BSWAP, POPCOUNT and ROTATE cases, BITREVERSE
can probably be fully expressed as DWARF code if need be,
but let's start with not throwing an internal error.

gcc:
	* dwarf2out.cc (mem_loc_descriptor): Handle BITREVERSE.