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.
Previously if a non-void function implictly returned, a dummy variable
was created as return value. Now instead it returns the result of the
OpUndef instruction. This better conveys the presence of undefined
behavior to SPIR-V consuming tools (and humans).
It also saves one ID per occurrence...
