Refactor RB_SPECIAL_CONST_P (#6759)

Since https://github.com/ruby/ruby/pull/6599, RUBY_IMMEDIATE_MASK also
overlaps RUBY_Qnil. Now RB_SPECIAL_CONST_P seems confusing since both
RB_IMMEDIATE_P and RB_TEST check for RUBY_Qnil while we only need to
check RUBY_Qnil besides RUBY_IMMEDIATE_MASK. I'd like to make this
change to make it less confusing.

I confirmed that this doesn't change the number of instructions used for
the RUBY_Qfalse check on Linux x86_64 GCC and macOS arm64 Clang.

include/ruby/internal/special_consts.h

@@ -326,7 +326,7 @@ RBIMPL_ATTR_ARTIFICIAL()
 static inline bool
 RB_SPECIAL_CONST_P(VALUE obj)
 {
-    return RB_IMMEDIATE_P(obj) || ! RB_TEST(obj);
+    return RB_IMMEDIATE_P(obj) || obj == RUBY_Qfalse;
 }
 
 RBIMPL_ATTR_CONST()