Fix KhronosGroup#1130

This addresses review feedback for the CCP implementation (which fixes
KhronosGroup#889).

This adds more protection around the folding of instructions that would
not be supported by the folder.

SPIRV-Tools v2017.2

SPIRV-Tools v2017.1

Finalize v2016.6

SPIR-V version 1.1 revision 1

Support SPIR-V 1.0 Rev3

Assembler, disassembler, binary parser are complete.
Validator is work in progress.

Assembler, Disassembler support for SPIR-V 0.99 Rev32

Does not support 16-bit float literals