[mlir][NFC] remove stray decl of toAffineExpr, rename for readability

Summary:
- remove stray toAffineExpr decl in affine analysis (name duplicate of
  mlir::toAffineExpr)

- rename mlir::toAffineExpr for better readability

- related NFC changes

Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>

Differential Revision: https://reviews.llvm.org/D75694