[PPCGCodeGeneration] Correct usage of llvm::Value with getLatestValue.

It is possible that the `HostPtr` that coresponds to an array could be
invariant load hoisted. Make sure we use the invariant load hoisted
value by using `IslNodeBuilder::getLatestValue`.

Differential Revision: https://reviews.llvm.org/D36001

llvm-svn: 309681