In preparation for implementing support for detection of non-protected
call instructions, refine the definition of state which is computed for
each register by data-flow analysis.
Explicitly marking the registers which are known to be trusted at
function entry is crucial for finding non-protected calls. In addition,
it fixes less-common false negatives for pac-ret, such as `ret x1` in
`f_nonx30_ret_non_auted` test case.
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