Convert check_is_double, check_both_double, and box_double_or_int32 to
declare their internal scratch as macro-local `temp` decls instead of
hardcoding t3/t4. Each invocation now gets a freshly uniquified temp
that the allocator places independently of any other live values in
the caller, so adding a `temp` declaration around a sequence that
calls into one of these macros no longer needs to know which
positional slot the macro happened to be using.
box_double_or_int32 in particular is on the hot arithmetic path
(Add/Sub/Mul/PostfixIncrement/etc.), so this opens the door to
migrating those handlers without conflicting with the macro's
internal scratch use.