swift234

Create RCIdentityAnalysis a cache for stripRCIdentityPreservingOps.

The cache is needed to ensure we do not run into compile time problems once we
start looking through Phi Nodes.

The analysis is currently disabled and just returns
SILValue::stripRCIdentityPreservingOps. I am going to thread it through the rest
of the passes that use that call. Then I am going to hide
stripRCIdentityPreservingArgs. Finally post OzU, I am going to enable the pass.

rdar://18300069

Swift SVN r21891