Drop unbounded identity map from MemRef affine map composition.

Unbounded identity maps do not affect the accesses through MemRefs in any way.
A previous CL dropped such maps only if they were alone in the composition.  Go
further and drop such maps everywhere they appear in the composition.

Update the parser test to check for unique'd hoisted map to be present but
without assuming any particular order.  Because some of the hoisted identity
maps still apear due to the nested "for" statements, we need to check for them.
However, they no longer appear above the non-identity maps because they are no
longer necessary for the extfunc memref declarations that are textually first
in the test file.  This order may change further as map simplification is
improved, there is no reason to assume a particular order.

PiperOrigin-RevId: 219287280