gallivm: fix opcode and function nesting

gallivm: fix opcode and function nesting

gallivm soa code supported only a single level of nesting for
control flow opcodes (if, switch, loops...) but the d3d10 spec
clearly states that those are nested within functions. To support
nesting of conditionals inside functions we need to store the
nesting data inside function contexts and keep a stack of those.
Furthermore we make sure that if nesting for subroutines is deeper
than 32 then we simply ignore all subsequent 'call' invocations.

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