[SelectionDAG] Don't subject ISD:Constant to the depth limit in...

[SelectionDAG] Don't subject ISD:Constant to the depth limit in TargetLowering::SimplifyDemandedBits.

Summary:
We shouldn't recurse any further but it doesn't mean we shouldn't be able to give the known bits for a constant. The caller would probably like that we always return the right answer for a constant RHS. This matches what InstCombine does in this case.

I don't have a test case because this showed up while trying to revive D31724.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: arsenm, llvm-commits

Differential Revision: https://reviews.llvm.org/D38967

llvm-svn: 316255