Implement fuzzing of instructions from the GLSL extension

[rayanht]

Is your feature request related to a problem? Please describe.
N/A

Describe the solution you'd like
SPIRVSmith should be able to generate SPIR-V code that imports the GLSL extension and uses instructions from that set.

Describe alternatives you've considered
N/A

Additional context
GLSL Extension spec: https://www.khronos.org/registry/SPIR-V/specs/unified1/GLSL.std.450.html