vect: Tweak dump messages for vector mode choice

vect: Tweak dump messages for vector mode choice

After vect_analyze_loop has successfully analysed a loop for
one base vector mode B1, it considers using following base vector
modes to vectorise an epilogue.  However, for VECT_COMPARE_COSTS,
a later mode B2 might turn out to be better than B1 was.  Initially
this comparison will be between an epilogue loop (for B2) and a main
loop (for B1).  However, in r11-6458 I'd added code to reanalyse the
B2 epilogue loop as a main loop, partly for correctness and partly
for better costing.

This can lead to a situation in which we think that the B2 epilogue
loop was better than the B1 main loop, but that the B2 main loop is
not better than the B1 main loop.  There was no dump message to say
that this had happened, which made it look like B2 had still won.

gcc/
	* tree-vect-loop.c (vect_analyze_loop): Print a dump message
	when a reanalyzed loop fails to be cheaper than the current
	main loop.