HLSL HS outputs a per ctrl point value, and the DS reads an array
of that type.  (It also has a per patch frequency).  The per-ctrl-pt
frequency is arrayed on just one side, as opposed to SPIR-V which
is arrayed on both.  To match semantics, the compiler creates an
array behind the scenes and indexes it by invocation ID, assigning
the HS return value to it.