Improve -fopt-info-vec for basic-block vectorization

We currently dump notes like

flow_lam.f:65:72: optimized: basic block part vectorized using 32 byte vectors
flow_lam.f:65:72: optimized: basic block part vectorized using 32 byte vectors
flow_lam.f:65:72: optimized: basic block part vectorized using 32 byte vectors
flow_lam.f:65:72: optimized: basic block part vectorized using 32 byte vectors
..

repeating the same location for multiple instances because we
clobber vect_location during BB vectorization.  The following
avoids this, improving things to

flow_lam.f:15:72: optimized: basic block part vectorized using 32 byte vectors
flow_lam.f:16:72: optimized: basic block part vectorized using 32 byte vectors
flow_lam.f:17:72: optimized: basic block part vectorized using 32 byte vectors
flow_lam.f:18:72: optimized: basic block part vectorized using 32 byte vectors
...

	* tree-vect-slp.cc (vect_slp_region): Save/restore vect_location
	around dumping code.