zink: handle ntv case of nested loop instructions more permissively

zink: handle ntv case of nested loop instructions more permissively

if the last instruction in a loop's body terminates a block, e.g., from
a nested loop with a jump as its final instruction, then no block will
have been started when returning to the original loop, and there's no need
to emit a branch

fixes shaders@glsl-vs-continue-in-switch-in-do-while

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5852>