[Bug rtl-optimization/93565] [9/10 regression] Combine duplicates instructions

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93565

--- Comment #22 from Segher Boessenkool <segher at gcc dot gnu.org> ---
                    T0        T2        T3        T4
       alpha   6049096  100.020%  100.018%  100.001%
         arc   4019384  100.000%   99.989%   99.989%
         arm  14177962   99.999%   99.999%  100.000%
       arm64  12968466   99.938%   99.888%  100.000%
         c6x   2346077  100.000%  100.001%  100.001%
        csky   3332454  100.000%  100.000%  100.000%
       h8300   1165256   99.999%   99.999%  100.000%
        i386  11227764  100.001%  100.001%  100.000%
        ia64  18088488  100.003%  100.007%  100.003%
        m68k   3716871  100.000%  100.000%  100.000%
  microblaze   4935181  100.000%   99.995%   99.995%
        mips   8407681  100.000%  100.000%  100.000%
      mips64   6979344   99.987%   99.981%   99.981%
       nds32   4471023  100.000%   99.994%   99.994%
       nios2   3643253  100.000%   99.999%   99.999%
    openrisc   4182200  100.000%   99.995%   99.995%
      parisc   7710095  100.001%  100.001%  100.000%
    parisc64   8676725  100.003%  100.002%   99.999%
     powerpc  10603859  100.000%  100.000%  100.001%
   powerpc64  17552718  100.007%  100.005%   99.999%
 powerpc64le  17552718  100.007%  100.005%   99.999%
     riscv32   1546172  100.000%   99.999%   99.999%
     riscv64   6623170  100.010%  100.005%  100.001%
        s390  13103095   99.995%   99.993%   99.999%
          sh   3216555   99.999%   99.992%   99.993%
     shnommu   1611176   99.999%   99.999%  100.000%
       sparc   4363333  100.000%   99.997%   99.997%
     sparc64   6751939  100.000%   99.997%   99.997%
      x86_64  19681173  100.000%  100.000%  100.000%
      xtensa         0         0         0         0

T0 is orig, T2 is only sign_extend, T3 is sign_extend and no same sources,
T4 is only no same source (SET_SRC).

The diffs look less than they are, this is just size, and with 2-2 combines
size does not change (on many targets).  For powerpc, *all* the changes
these patches make hurt code quality (they change two parallel insns to
two sequential ones).

I think combine should just do what it already does, and you should add
some peepholes, or maybe some new pass?