gallium/aux/tgsi_sanity.c: Fix -Wsign-compare warnings

gallium/aux/tgsi_sanity.c: Fix -Wsign-compare warnings

tgsi_sanity.c: In function 'iter_instruction':
tgsi_sanity.c:316:29: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
       if (ctx->index_of_END != ~0) {
                             ^~
tgsi_sanity.c: In function 'epilog':
tgsi_sanity.c:488:26: warning: comparison between signed and unsigned
integer expressions [-Wsign-compare]
    if (ctx->index_of_END == ~0) {

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>