[mlir][sparse] Fix a problem in handling data type conversion.

Previously, the genCast function generates arith.trunci for converting f32 to
i32. Fix the function to use mlir::convertScalarToDtype to correctly handle
conversion cases beyond index casting.

Add a test case for codegen the sparse_tensor.convert op.

Reviewed By: aartbik, Peiming, wrengr

Differential Revision: https://reviews.llvm.org/D147272