tgsi/scan: correctly walk instructions in tgsi_scan_tess_ctrl()

tgsi/scan: correctly walk instructions in tgsi_scan_tess_ctrl()

The previous code used a do while loop and continues after walking
a nested loop/if-statement. This means we end up evaluating the
last instruction from the nested block against the while condition
and potentially exit early if it matches the exit condition of the
outer block.

Fixes: 386d165d8d09 ("tgsi/scan: add a new pass that analyzes tess factor writes")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>