This allows two variables of the same struct type to be flattened
into the same interface struct without a member name conflict.
Add shaders-msl/frag/in_block_with_multiple_structs_of_same_type.frag
unit test shader to demonstrate this.
To support loading array of array properly in tessellation, we need a
rewrite of how tessellation access chains are handled.
The major change is to remove the implicit unflatten step inside
access_chain which does not take into account the case where you load
directly from a control point array variable.
We defer unflatten step until OpLoad time instead.
This fixes cases where we load array of {array,matrix,struct}.
Removes the hacky path for MSL access chain index workaround.
