Fix #137 by avoiding a C++11 feature.

SPV: Split loop header from condition testing for for/while loops.

Apparently, older MSVC versions don't support brace-initializers for
function arguments.

Thanks @baldurk for a suggestion on his branch.

SPV: Don't remove SPIR-V blocks before codegen, use new InReadableOrder instead.

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.

SPV: Rework loop code generation to match SPIR-V 1.0.

SPV: Code-generate SPIR-V blocks in a more readable order

Add a test for unreachable merge block.

Update test results with the new order: mainly delaying merge blocks and
removing unreachable ones.

Freed up some command line memory (not used programmatically).

Free memory associated with SPIR-V generation and the pragmaTable.