TargetLowering: Add SimplifyDemandedBits() helper to TargetLoweringOpt

Summary:
The main purpose of this new helper is to enable simplifying operations that
have multiple uses.  SimplifyDemandedBits does not handle multiple uses
currently, and this new function makes it possible to optimize:

and v1, v0, 0xffffff
mul24 v2, v1, v1      ; Multiply ignoring high 8-bits.

To:

mul24 v2, v0, v0

Where before this would not be optimized, because v1 has multiple uses.

Reviewers: bogner, arsenm

Subscribers: nhaehnle, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D24964

llvm-svn: 284266