st/glsl_to_tgsi: fix a use-after-free in merge_two_dsts

st/glsl_to_tgsi: fix a use-after-free in merge_two_dsts

Found by address sanitizer.

The loop here tries to be safe, but in doing so, it ends up doing
exactly the wrong thing: the safe foreach is for when the loop
variable (inst) could be deleted and nothing else. However, this
particular can delete inst's successor, but not inst itself.

Fixes: 8c6a0ebaad72 ("st/mesa: add st fp64 support (v7.1)")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>