AMDGPU: mark SET_INACTIVE_* as convergent operation

set_inactive is actually a kind of operation that is passing certain
value from active threads to inactive threads. In later WWM operation,
the activated threads which were disabled before would read such
values passed to them by set_inactive operation. So I think the
set_inactive is a convergent operation.

Differential Revision: https://reviews.llvm.org/D147683