Drop trivial identity affine mappings in MemRef construction.

As per MLIR spec, the absence of affine maps in MemRef type is interpreted as
an implicit identity affine map.  Therefore, MemRef types declared with
explicit or implicit identity map should be considered equal at the MemRefType
level.  During MemRefType construction, drop trivial identity affine map
compositions.  A trivial identity composition consists of a single unbounded
identity map.  It is unclear whether affine maps should be composed in-place to
a single map during MemRef type construction, so non-trivial compositions that
could have been simplified to an identity are NOT removed.  We chose to drop
the trivial identity map rather than inject it in places that assume its
present implicitly because it makes the code simpler by reducing boilerplate;
identity mappings are obvious defaults.

Update tests that were checking for the presence of trivial identity map
compositions in the outputs.

PiperOrigin-RevId: 218862454