EXTEND_HWM_SET() should not declare "ix"

[hvds]

We need to be a lot more careful in macros to avoid using variable names likely to appear in the interpolated arguments. See https://perlmonks.org/?node_id=11144355 for an example, which I reckon was unreasonably hard to debug:

                EXTEND(SP, ix == 3 ? 2*n : n);

I've no idea what the compiler is actually trying to do with this in:

#  define EXTEND_HWM_SET(p, n)                      \
        STMT_START {                                \
            SSize_t ix = (p) - PL_stack_base + (n); \
            if (ix > PL_curstackinfo->si_stack_hwm) \
                PL_curstackinfo->si_stack_hwm = ix; \
        } STMT_END
#  define EXTEND(p,n)   STMT_START {                                    \
                         EXTEND_HWM_SET(p, n);                          \
                         if (UNLIKELY(_EXTEND_NEEDS_GROW(p,n))) {       \
                           sp = stack_grow(sp,p,_EXTEND_SAFE_N(n));     \
                           PERL_UNUSED_VAR(sp);                         \
                         } } STMT_END

but I suspect we're in nasal demon territory.

[hvds]

This has been solved in blead by merging #19819, does this need to be backported for users of perl < 5.38 to benefit from it, or can it be adressed via ppport.h?