Use 'location_hash' for 'seen_locations' in 'gcc/profile.c:branch_prob'

Follow-up to commit 102fcf94
"Fix GCOV CFG related issues": considering the current
'int_hash <location_t, 0, 2>', per 'libcpp/include/line-map.h':

      Actual     | Value                         | Meaning
      -----------+-------------------------------+-------------------------------
      0x00000000 | UNKNOWN_LOCATION (gcc/input.h)| Unknown/invalid location.
      -----------+-------------------------------+-------------------------------
      0x00000001 | BUILTINS_LOCATION             | The location for declarations
                 |   (gcc/input.h)               | in "<built-in>"
      -----------+-------------------------------+-------------------------------
      0x00000002 | RESERVED_LOCATION_COUNT       | The first location to be
                 | (also                         | handed out, and the
                 |  ordmap[0]->start_location)   | first line in ordmap 0

... this currently uses value '0' ('UNKNOWN_LOCATION') as spare values for
'Empty', and value '2' ('RESERVED_LOCATION_COUNT') as spare values for
'Deleted', which is questionable?

What actually does get put into 'seen_locations' is (mostly...)
restricted/gated by '!RESERVED_LOCATION_P' (which is true unless
'UNKNOWN_LOCATION' or 'BUILTINS_LOCATION'), thus we may simply use
'location_hash'.

	gcc/
	* profile.c (branch_prob): Use 'location_hash' for
	'seen_locations'.