[Bug target/109632] Inefficient codegen when complex numbers are emulated with structs

["rsandifo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109632

--- Comment #10 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> ---
After prototyping this further, I no longer think that lowering
at the gimple level is the best answer.  (I should have listened
to Richi.)  Although it works, its major drawback is that
it's one-sided: it allows the current function's PARM_DECLs
and returns to be lowered to individual scalars, but it does
nothing for calls to other functions.  Being one-sided means
(a) that lowering only solves half the problem and (b) that tail
calls cannot be handled easily after lowering.

One thing that does seem to work is to force the structure to have
V2SF (and fix the inevitable ABI fallout).  That could only be done
conditionally, based on a target hook.  But it seems to fix both
test cases: the pass-by-reference one and the pass-by-value one.