Mark CONV as non-weak, to prevent elimination of its side-effect.

An unused guarded CONV int.num cannot be omitted in general.

src/lj_ir.h

@@ -126,7 +126,7 @@
   _(XBAR,	S , ___, ___) \
   \
   /* Type conversions. */ \
-  _(CONV,	NW, ref, lit) \
+  _(CONV,	N , ref, lit) \
   _(TOBIT,	N , ref, ref) \
   _(TOSTR,	N , ref, ___) \
   _(STRTO,	N , ref, ___) \