[mlir][Linalg] Avoid using scf.parallel for non-parallel loops in Linalg ops.

Modifying the loop nest builder for generating scf.parallel loops to
not generate scf.parallel loops for non-parallel iterator types in
Linalg operations. The existing implementation incorrectly generated
scf.parallel for all tiled loops. It is rectified by refactoring logic
used while lowering to loops that accounted for this.

Differential Revision: https://reviews.llvm.org/D80188