nir/search: Don't match inexact expressions with exact subexpressions

nir/search: Don't match inexact expressions with exact subexpressions

In the first pass of implementing exact handling, I made a mistake with
search-and-replace.  In particular, we only reallly handled exact/inexact
on the root of the tree.  Instead, we need to check every node in the tree
for an exact/inexact match.  As an example of this, consider the following
GLSL code

precise float a = b + c;
if (a < 0) {
   do_stuff();
}

In that case, only the add will be declared "exact" and an expression that
looks for "b + c < 0" will still match and replace it with "b < -c" which
may yield different results.  The solution is to simply bail if any of the
values are exact when matching an inexact expression.