Remove splay_tree from gimplify.c

[Jakub Jelinek]

On Wed, May 27, 2015 at 10:34:46AM -0600, Jeff Law wrote:
> So the question here is whether or not the other uses are commonly looking
> up elements we've already searched for -- that's the whole purpose of a
> splay tree, to improve lookup performance for commonly hit items.

First of all, this is only used for OpenMP/OpenACC/Cilk+ constructs,
so it really isn't that performance criticial.
The code probably dates back to Richard's and Diego's changes.
And, I believe splay trees are the right thing to use here, while sometimes
you lookup different vars, looking up the same var many times in a row is
very common.

	Jakub