nir/builder: Short-circuit in nir_type_convert if no conversion happens

nir/builder: Short-circuit in nir_type_convert if no conversion happens

If both types are the same or both are integer types with the same bit
size, no actual conversion happens and nir_type_conversion_op() will
return nir_op_mov.  In this case, there's no point in emitting the move
and we can just return src instead.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20067>