[mlir][sparse] Put the implementation for the insertion operation to subroutines.

Previously, we generated inlined implementation for insert operation and
observed MLIR compile time increase due to the size of the main routine. We now
put the insert operation implementation in subroutines and leave the inlining
decision to the MLIR compiler.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D138957