[ScopInfo] Introduce ScopStmt::getSurroundingLoop(). NFC.

Introduce ScopStmt::getSurroundingLoop() to replace getFirstNonBoxedLoopFor.

getSurroundingLoop() returns the precomputed surrounding/first non-boxed
loop. Except in ScopDetection, the list of boxed loops is only used to
get the surrounding loop. getFirstNonBoxedLoopFor also requires LoopInfo
at every use which is not necessarily available everywhere where we may
want to use it.

Differential Revision: https://reviews.llvm.org/D30985

llvm-svn: 297899