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.