[PATCH] Fix DCE REG_LIBCALL note moving from noop move insns (PR rtl-optimization/33644)

[Richard Kenner]

> We'd certainly need something to say that a function call instruction is
> deletable if its value is unused; in general, of course, we assume
> function calls have important side-effects.

I thought we have a flag for that now, but I'm not sure I remember which one.

I agree that it may well be the case that libcalls are ALREADY unnecessary,
but I'm not quite ready to assert that myself.