Avoid quadratic behaviour in prune_runtime_alias_test_list

Avoid quadratic behaviour in prune_runtime_alias_test_list

prune_runtime_alias_test_list used ordered_remove to remove a merged
alias pair, which made the function quadratic when many aliases could
be removed.

I had a testcase in which these memmoves accounted for an impressive
85% of compile time.  The fact that we had so many probably shows
a deeper problem, but still, it's easy to remove as we go.

2019-12-06  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-data-ref.c (prune_runtime_alias_test_list): Exit early
	for empty vectors.  Avoid using ordered_remove and instead
	shuffle the vector as we go.

From-SVN: r279038