Post-RA scheduler compile time fix. Quadratic computation of DAG node depth.

Post-RA scheduler compile time fix. Quadratic computation of DAG node depth.

The post-ra scheduler was explicitly updating the depth of a node's
successors after scheduling it, regardless of whether the successor
was ready. This is quadratic for DAGs with transitively redundant
edges. I simply removed the useless update of depth, which is lazilly
computed later.
Fixes <rdar://problem/9044332> compiler takes way too long to build TextInput.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130992 91177308-0d34-0410-b5e6-96231b3b80d8