testsuite: Enable the PR 87600 tests for LoongArch
I'm going to refine a part of the PR 87600 fix which seems triggering
PR 120983 that LoongArch is particularly suffering. Enable the PR 87600
tests so I'll not regress PR 87600.
gcc/testsuite/ChangeLog:
PR rtl-optimization/87600
PR rtl-optimization/120983
* gcc.dg/pr87600.h [__loongarch__]: Define REG0 and REG1.
* gcc.dg/pr87600-1.c (dg-do): Add loongarch.
* gcc.dg/pr87600-2.c (dg-do): Likewise.
(cherry picked from commit
651845ceaae26a1b2c03f215e16e7e711dcea2cc)