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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@262844 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 7e94e7eb7efc0adc2c459025b4d4f3622cf4a25c)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm