ir3/ra: Fix logic bug in compress_regs_left
If we're allocating a source then we force is_killed to false, not to true. Fixes a regression in dEQP-GLES31.functional.synchronization.in_invocation.image_atomic_write_read later. Fixes: 0ffcb19b ("ir3: Rewrite register allocation") Part-of: <mesa/mesa!14246> (cherry picked from commit d371d807)