Teach SCEV to look through returned-argument functions

When building SCEVs, if a function is known to return its argument, then we can
build the SCEV using the corresponding argument value.

Differential Revision: http://reviews.llvm.org/D9381

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275037 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed