[LoopDataPrefetch] If prefetch distance is not set, skip pass

This lets select sub-targets enable this pass.  The patch implements the
idea from the recent llvm-dev thread:
http://thread.gmane.org/gmane.comp.compilers.llvm.devel/94925

The goal is to enable the LoopDataPrefetch pass for the Cyclone
sub-target only within Aarch64.

Positive and negative tests will be included in an upcoming patch that
enables selective prefetching of large-strided accesses on Cyclone.

llvm-svn: 262844