[SCEV] Take correct loop in AddRec simplification. PR40420

The code of AddRec simplification is using wrong loop when it creates a new
AddRecExpr. It should be using AddRecLoop which we have saved and against which
all gate checks are made, and not calling AddRec->getLoop() over and over
again because AddRec may change and become an AddRecurrency from outer loop
during the transform iterations.

Considering this change trivial, commiting for postcommit review.

llvm-svn: 352451