testsuite: Adjust bb-slp-pr68892.c for AArch64

testsuite: Adjust bb-slp-pr68892.c for AArch64

AArch64 passes the "not profitable" test because it treats vec_construct
as having a high-enough cost.  This means that we can try other vector
modes, which in turn causes "BB vectorization with gaps at the end of
a load is not supported" to be printed more than once.  The number of
times that we print the message doesn't seem important, so the patch
converts it to a plain scan-tree-dump.

gcc/testsuite/
	* gcc.dg/vect/bb-slp-pr68892.c: Don't XFAIL the profitability
	test for aarch64*-*-*.  Allow the "BB vectorization with gaps"
	message to be printed more than once.