[mlir][index] Implement folder for CastSOp and CastUOp

[jymchng]

Attention to: @Mogball

This is a follow-up of the ticket raised here on Mojo's Github repo.

array3.mojo:13:64: note:               failed to fold operation index.castu(4 : index)
         value: __mlir_op.`index.castu`[_type: __mlir_type.ui8](value_as_index)

The error message shows that fold operation has failed for unsigned casting to inbuilt type ui8 from type index.

[llvmbot]

@llvm/issue-subscribers-mlir

Attention to: @Mogball

This is a follow-up of the ticket raised here on Mojo's Github repo.

array3.mojo:13:64: note:               failed to fold operation index.castu(4 : index)
         value: __mlir_op.`index.castu`[_type: __mlir_type.ui8](value_as_index)

The error message shows that fold operation has failed for unsigned casting to inbuilt type ui8 from type index.