[Invariant Loads] Do not consider invariant loads to have dependences.

We need to relax constraints on invariant loads so that they do not
create fake RAW dependences. So, we do not consider invariant loads as
scalar dependences in a region.

During these changes, it turned out that we do not consider `llvm::Value`
replacements correctly within `PPCGCodeGeneration` and `ISLNodeBuilder`.
The replacements dictated by `ValueMap` were not being followed in all
places. This was fixed in this commit. There is no clean way to decouple
this change because this bug only seems to arise when the relaxed
version of invariant load hoisting was enabled.

Differential Revision: https://reviews.llvm.org/D35120

llvm-svn: 307907