Limit the recursion depth of SelectionDAG::isSplatValue()

This method previously always recursively checked both the left-hand
side and right-hand side of binary operations for splatted (broadcast)
vector values to determine if the parent DAG node is a splat.

Like several other SelectionDAG methods, limit the recursion depth to
MaxRecursionDepth (6). This prevents stack overflow.
See also https://issuetracker.google.com/173785481

Patch by Nicolas Capens. Thanks!

Differential Revision: https://reviews.llvm.org/D92421