draw/llvm: fix generation of the VS with GS present

draw/llvm: fix generation of the VS with GS present

draw_current_shader_* functions return a final output when considering
both the geometry shader and the vertex shader. But when code generating
vertex shader we can not be using output slots from the geometry shader
because, obviously, those can be completely different. This fixes a
number of very non-obvious crashes.
A side-effect of this bug was that sometimes the vertex shading code
could save some random outputs as position/clip when the geometry
shader was writing them and vertex shader had different outputs at
those slots (sometimes writing garbage and sometimes something correct).

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Matthew McClure <mcclurem@vmware.com>