[Bug rtl-optimization/82683] Combine: GCC generates bad code with -tune=thunderx2t99

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82683

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Ah.  So we start with

insn_cost 4 for    18: r91:DI=r83:DI<<0x2
      REG_DEAD r83:DI
insn_cost 4 for    19: r78:DI=r76:DI+r91:DI
      REG_DEAD r91:DI
      REG_DEAD r76:DI
insn_cost 20 for    20: r82:SI=[r78:DI]
      REG_DEAD r78:DI
insn_cost 4 for    21: r83:DI=sign_extend(r82:SI)

and first 18,19 are combined, and then 19,20,21.  Both 19 and 21 set
r83, it is dead after 19, but that note is put on 21.