gallivm/soa: implement indirect addressing in immediates

gallivm/soa: implement indirect addressing in immediates

The support is analogous to the way we handle indirect addressing
in temporaries, except that we don't have to worry about storing
(after declarations) and thus we'll able to keep using the old
code when indirect addressing isn't used. In other words we're
still using constants directly, unless the instruction has
immediate register with indirect addressing.

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