glsl: Fix merging of layout(invocations) with other qualifiers

If another layout qualifier appeared to the left of `invocations` in the
GS input layout declaration, the invocation count would be dropped on
the floor.

Fixes the piglit tests:

spec/ARB_transform_feedback3/arb_transform_feedback3-ext_interleaved_two_bufs_gs_max
spec/ARB_gpu_shader5/arb_gpu_shader5-invocation-id
spec/ARB_gpu_shader5/compiler/correct-multiple-layout-qualifier-invocations.geom
spec/ARB_gpu_shader5/execution/invocations-conflicting

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Tested-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>