[ValueTracking] peek through 2-input shuffles in ComputeNumSignBits

This patch gives the IR ComputeNumSignBits the same functionality as the 
DAG version (the code is derived from the existing code).

This an extension of the single input shuffle analysis added with D53659.

Differential Revision: https://reviews.llvm.org/D53987


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346071 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed