[SCEV] Fix issues found during the review of r251283. NFC.

Summary:
Replace (const SCEVAddRecExpr *) with cast<SCEVAddRecExpr>.

Rename SCEVApplyRewriter to SCEVLoopAddRecRewriter (which is a more
appropriate name) since the description is "takes a scalar evolution
expression and applies the Map (Loop -> SCEV) to all AddRecExprs."

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D14065

llvm-svn: 251292