[Simplify] Add -polly-simplify pass.

This new pass removes unnecessary accesses and writes. It currently
supports 2 simplifications, but more are planned.

It removes write accesses that write a loaded value back to the location
it was loaded from. It is a typical artifact from DeLICM. Removing it
will get rid of bogus dependencies later in dependency analysis.

It also removes statements without side-effects. ScopInfo already
removes these, but the removal of unnecessary writes can result in
more side-effect free statements.

Differential Revision: https://reviews.llvm.org/D30820

llvm-svn: 297473