Q's about use of is_gimple_reg_rhs/is_gimple_tmp_var

[Richard Kenner]

    Right.  That would be a possible optimization.  Forcing the LHS to be
    a temporary ensures the condition that were is no other assignment to
    the variable.  If you have a clean way to test that this is the only
    assignment to the RHS, then sure, you can avoid the temporary copy.

    I'm not sure how useful that is since the copy propagator ought to do
    the right thing in the case you're considering.

I'm not thinking of it as a "possible optimization", but instead trying to
pin down the exact condition that gimplifications needs to apply to the
RHS.  As I said, I think that's "assigned only once".