[LoopAccessAnalysis] Add a const qualifier to getMaxSafeDepDistBytes()

Add a const qualifier to this API call, since this is a member of
MemoryDepChecker and LoopAccessInfo returns an object of this class as a
const, as follows:

const MemoryDepChecker &getDepChecker() const { return *DepChecker; }

If one tries to use function as follows:

LAI->getDepChecker().getMaxSafeDepDistBytes()

results in the following error:

passing ‘const llvm::MemoryDepChecker’ as ‘this’ argument discards
qualifiers

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D156304