Allow irinterp to refine nothrow effect (#48066)
This addresses a remaining todo in the irinterp code to allow it
to compute whether its particular evaluation refined `nothrow`.
As a result, we can re-enable it for a larger class of ir (we had
previously disabled it to avoid regressing cases where regular
constprop was able to prove a `nothrow` refinement, but irinterp
was not).
