Small optimize to `pp_int` in pp.c

[t-a-k]

Similar to commit 75122876d2e240d7aafe35db62c52dc639af3437, SvUV_nomg(sv) in pp_int in pp.c could be safely replaced with simple SvUVX(sv), as SvIOK(sv) and SvIsUV(sv) are already tested.

This patch should not change any behavior of perl (merely very slight performance optimization), so I think it does not require a perldelta entry.