[DAG] Add SelectionDAG::MaxRecursionDepth constant

[DAG] Add SelectionDAG::MaxRecursionDepth constant

As commented on D67557 we have a lot of uses of depth checks all using magic numbers.

This patch adds the SelectionDAG::MaxRecursionDepth constant and moves over some general cases to use this explicitly.

Differential Revision: https://reviews.llvm.org/D67711

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372315 91177308-0d34-0410-b5e6-96231b3b80d8