testsuite: Ignore line no. for BB vectorization message

testsuite: Ignore line no. for BB vectorization message

The case g++.dg/vect/slp-pr56812.cc need to be fixed a bit with
line number neglection since the message for basic block vectorization
looks like:

  slp-pr56812.cc:19:1: optimized: basic block part vectorized using
  16 byte vectors

while for loop vectorization, it looks like:

  slp-pr56812.cc:17:18: optimized: loop vectorized using 16 byte
  vectors

gcc/testsuite/ChangeLog:

	* g++.dg/vect/slp-pr56812.cc: Ignore line number for basic block
	vectorization messages.