[MLIR] Add folder for zero trip count affine.for

AffineForOp's folding hook is expected to fold away trivially empty
affine.for.  This allows simplification to happen as part of the
canonicalizer and from wherever the folding hook is used. While more
complex analysis based zero trip count detection is available from other
passes in analysis and transforms, simple and inexpensive folding had
been missing.

Also, update/improve affine.for op documentation clarifying semantics of
the result values for zero trip count loops.

Differential Revision: https://reviews.llvm.org/D106123