pretty-print SSA names

This adds the SSA name version to the gdb pretty-printing of SSA names.

(gdb) p (tree)$1
$5 = <ssa_name 0x7ffff68435a0 323>

2020-04-16  Richard Biener  <rguenther@suse.de>

	* gdbhooks.py (TreePrinter): Print SSA_NAME_VERSION of SSA_NAME
	nodes.