rs6000: Allow FPRs to change between SDmode and DDmode [PR94254]

rs6000: Allow FPRs to change between SDmode and DDmode [PR94254]

    g:497498c878d48754318e486428e2aa30854020b9 caused lra to cycle
    on some SDmode reloads for power6.  As explained in more detail
    in the PR comments, the problem was a conflict between two target
    hooks: rs6000_secondary_memory_needed_mode required SDmode FPR
    reloads to use DDmode memory (rightly, since using SDmode memory
    wouldn't make progress) but rs6000_can_change_mode_class didn't
    allow FPRs to change from SDmode to DDmode.  Previously lra
    ignored that and changed the mode anyway.

    From what Segher says, it sounds like the "from_size < 8 || to_size < 8"
    check is mostly there for SF<->64-bit subregs, and that SDmode is stored
    in the way that target-independent code expects.  This patch therefore
    allows SD<->DD changes.

    I wondered about checking for SD<->64-bit changes instead, but that
    seemed like an unnecessary generalisation for this stage.

    2020-03-23  Richard Sandiford  <richard.sandiford@arm.com>

    gcc/
            PR target/94254
            * config/rs6000/rs6000.c (rs6000_can_change_mode_class): Allow
            FPRs to change between SDmode and DDmode.