Previously, casting an object of a struct type to an identical type
would produce an error. This PR allows this case.
As a side-effect of the change, several self-type casts in existing
tests go away. For example:
0:10 Construct float ( temp float)
0:10 'f' ( in float)
becomes this (without the unneeded constructor op):
0:10 'f' ( in float)
For vector or array types this can result in somewhat less overall code.
Fixes: #1218
