Prior to this patch, when `NumElems` was 0, `OpTypeRuntimeArray` was
directly generated, but it requires `Shader` capability, so it can only
be generated if `Shader` env is being used. We have observed a pattern
of using unbound arrays that translate into `[0 x ...]` types in OpenCL,
which implies `Kernel` capability, so `OpTypeRuntimeArray` should not be
used. To prevent this scenario, this patch simplifies GEP instructions
where type is a 0-length array and the first index is also 0. In such
scenario, we effectively drop the 0-length array and the first index.
Additionally, the newly added test prior to this patch was generating a
module with both `Shader` and `Kernel` capabilities at the same time,
but they're incompatible. This patch also fixes that.
Finally, prior to this patch, the newly added test was adding `Shader`
capability to the module even with the command line flag
`--avoid-spirv-capabilities=Shader`. This patch also has a fix for that.
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