nv50/ir: add support for gl_PrimitiveIDIn

nv50/ir: add support for gl_PrimitiveIDIn

Note that the primitive id is stored in a[0x18], while usually the
geometry instructions are of the form a[$a1 + 0x4] which gets mapped to
p[] space. We need to avoid the change from a[] to p[] here, so it's
keyed on whether the access is indirect or not.

Note that there's also a use-case for accessing e.g. a[$r1], however
that's not supported for now. (Could be added by checking the register
file of the indirect parameter.)

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>