These capabalities were added on declaration of the members, but
that is considered too aggressive, as those members are automatically
declared in some shaders that don't use them. Now, actual access
is needed to make the capabalities be declared.
A removed block releases its instructions, so Module::idToInstruction
suddenly contains dangling references. The original motivation for
block removal was to skip some unreachable blocks, but that's already
achieved by InReadableOrder.cpp.
Also updated stale comments.
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.
