gallivm: Optimize emit_read_invocation's first-invocation loop.

gallivm: Optimize emit_read_invocation's first-invocation loop.

We don't need to deref invoc inside -- invoc is uniform in active
channels, so we can find our first active invocation in the loop, and then
dereference invocation once outside.

50 -> 46 seconds on
dEQP-VK.subgroups.ballot_broadcast.compute.subgroupbroadcast_i16vec4

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21001>