[Bug target/94393] Powerpc suboptimal 64-bit constant comparison

[amodra at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94393

--- Comment #3 from Alan Modra <amodra at gmail dot com> ---
There are two parts to fixing this PR.
1) We can do better in the sequences generated for some constants.
2) rs6000_rtx_costs needs to be accurate, so that expensive constants are put
in memory and stay there with optimisation.

Having looked at part 2 a little, I'd say fixes for that are definitely not
stage 4 material.