[mlir][sparse] refactoring: move genAffine to loopEmitter

[mlir][sparse] refactoring: move genAffine to loopEmitter

This is the beginning patch of a sequence of dependent patches that in together provide the affine expression on matched indexing mapping for sparse tensors.

This patch itself simply move `genAffine` into loop emitter to be prepared for upcoming patches.

D138169 provides support for affine expression on dense dimensions only (except for constant affine expression)
D138170 provides support for constant affine expressions on dense dimensions
D138171 provides **merger** support for affine expression on sparse dimension (without codegen)
D138172 provides **codegen** support (by generating a "filter" loop) for affine expression on sparse dimensions.
D138173 fixes a crash on resolveCycle when dealing with affine expressions.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D138168