regrename: Handle MEM segments

regrename: Handle MEM segments

This patch fixes the register rename pass to allow for MEM that have "segment"
elements in the address.

rtl.h and rtanal.h allow MEM addresses to have the format "segment + base +
offset", where "segment" is an arbitrary UNSPEC.  Without the patch, regrename
misidentifies the base and can break 64-bit addresses, on targets with 32-bit
registers, by placing the high-part in a non-contiguous register.

gcc/ChangeLog:

	* regrename.cc (scan_rtx_address): Recurse for segment UNSPECs.