[SCEV][NFC] Reduce memory footprint & compile time via DFS refactoring

Current implementations of DFS in SCEV check unique-visited of traversed
values on pop, and not on push. As result, the same value may be pushed
multiple times just to be thrown away when popped. These operations are
meaningless and only waste time and increase memory footprint of the
worklist.

This patch reworks the DFS strategy to check uniqueness before push.
Should be NFC.

Differential Revision: https://reviews.llvm.org/D111774
Reviewed By: nikic, reames