[NFC] [IslNodeBuilder, GPUNodeBuilder] Unify mechanism for looking up replacement Values.

We populate `IslNodeBuilder::ValueMap` which contains replacements for
`llvm::Value`s. There was no simple method to pick up a replacement if
it exists, otherwise fall back to the original.

Create a method `IslNodeBuilder::getLatestValue` which provides this
functionality.

This will be used in a later patch to fix bugs in `PPCGCodeGeneration`
where the latest value is not being used.

Differential Revision: https://reviews.llvm.org/D36000

llvm-svn: 309674