Fix non RUBY_DEBUG build warnings

On non RUBY_DEBUG builds, assert() compiles to nothing and the compiler
warns about uninitialized variables in those code paths. Replace
those asserts with rb_bug() to fix the warnings and do the assert in
all builds. Since yjit_asm_tests.c compiles outside of Ruby, it needed
a distinct version of rb_bug().

Also put YJIT_STATS check for function delcaration that is only defined
in YJIT_STATS builds.

yjit_codegen.c

@@ -1455,7 +1455,7 @@ jit_chain_guard(enum jcc_kinds jcc, jitstate_t *jit, const ctx_t *ctx, uint8_t d
         target0_gen_fn = gen_jbe_to_target0;
         break;
       default:
-        RUBY_ASSERT(false && "unimplemented jump kind");
+        rb_bug("yjit: unimplemented jump kind");
         break;
     };