HLSL: allow self-type cast (as no-op passthrough)

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

gtests/Hlsl.FromFile.cpp

@@ -315,6 +315,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.semicolons.frag", "main"},
         {"hlsl.shapeConv.frag", "main"},
         {"hlsl.shapeConvRet.frag", "main"},
+        {"hlsl.self_cast.frag", "main"},
         {"hlsl.snorm.uav.comp", "main"},
         {"hlsl.staticMemberFunction.frag", "main"},
         {"hlsl.store.rwbyteaddressbuffer.type.comp", "main"},