vartrack: fix skipping of unsuitable locs

vartrack: fix skipping of unsuitable locs

When the last loc in a chain happens to be an unsuitable_loc(), we
record VAR_LOC_FROM as if the empty location had been derived from
that unsuitable loc, instead of leaving it empty, which ends up
preventing the VAR loc from being recomputed when other related locs
are usefully expanded.

Adjust the code that skips an unsuitable_loc() to take the same
preparation steps we take for exiting the loop as when an expansion
attempt yields NULL_RTX.

for  gcc/ChangeLog

	* var-tracking.cc (vt_expand_var_loc_chain): Prepare to exit
	the loop after unsuitable_loc.