[Polly] Avoid use of `getStmtFor(BB)` in PolyhedralInfo. NFC

Summary: Since there will be no more a 1-1 correspondence between statements and basic block, we would like to get rid of the method `getStmtFor(BB)` and its uses. Here we remove one of its uses in PolyhedralInfo, as suggested by Michael Sir.

Reviewers: grosser, Meinersbur, bollu

Reviewed By: grosser

Subscribers: pollydev

Tags: #polly

Differential Revision: https://reviews.llvm.org/D35300

llvm-svn: 308220