gl_shader_decompiler: Guard out of bound geometry shader input reads

Geometry shaders follow a pattern that results in out of bound reads.
This pattern is:
- VSETP to predicate
- Use that predicate to conditionally set a register a big number
- Use the register to access geometry shaders
At the time of writing this commit I don't know what's the intent of
this number. Some drivers argue about these out of bound reads. To avoid
this issue, input reads are guarded limiting reads to the highest
posible vertex input of the current topology (e.g. points to 1 and
triangles to 3).