nir/spirv: Set a default number of invocations for geometry shaders
nir/spirv: Set a default number of invocations for geometry shaders

The SPIR-V spec says geometry shaders are supposed to have one invocation
by default.  The execution mode is only required if there are multiple
invocations.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler