[mlir] Refactor affine loop nest builders

Existing implementation of affine loop nest builders relies on EDSC
ScopedContext, which is not used pervasively. Provide a common OpBuilder-based
helper function to construct a perfect nest of affine loops with the body of
the innermost loop populated by a callback. Use this function to implement the
EDSC version.

Affine "for" loops differ from SCF "for" loops by (1) not allowing to yield
values and (2) supporting short-hand form for constant bounds, which justifies
a separate implementation of the loop nest builder for the same of simplicity.

Differential Revision: https://reviews.llvm.org/D81955