[Dominators] Make slow walks shorter

Summary:
When DFS numbers are not yet calculated for a dominator tree, we have to walk it up to say whether one node dominates some other.

This patch makes the slow walks shorter by only walking until the level of the node we check against is reached. This is because a node cannot possibly dominate something higher in its tree.

When running opt with -O3, the patch results in:
* 25% fewer loop iterations for `opt` (fullLTO)
* 30% fewer loop iterations for sqlite

Reviewers: brzycki, asbirlea, chandlerc, NutshellySima, grosser

Reviewed By: NutshellySima

Subscribers: mehdi_amini, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D49955

llvm-svn: 338396