[Bug target/103568] sub-optimal vector construction with two loaded doubles on Power10

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103568

--- Comment #1 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Confirmed.

This is cheaper in GPRs until we had the lxvrdx insn, which is power10 (ISA
3.1)
itself.

Wrt dword 1 being zeroed by fp insns.  ISA 3.1 has a note saying this was true
on all earlier implementations (namely, p7, p8, p9) as well.  As far as I know
these are in fact the only older implementations of VSX.