Before, it was only including explicit interface, sufficient for IO-Block-declared
oriented interface, but not sufficient for all modes GLSL might be used with
SPIR-V.
Two things are accomplished now:
1) each id will appear exactly once
2) the OpEntryPoint list will union static use with declarations
Structured control-flow rules allow leaving the middle of a construct through
a return, but not through a jump to a block that does a return.
Addresses issue #58.
