Perl_sv_vcatpvfn_flags: move Inf handling for ints

Perl_sv_vcatpvfn_flags: move Inf handling for ints

integer-like format types handle Inf/Nan specially. Currently the code to
handle this in the main execution path, guarded by

    if (strchr("BbcDdiOouUXx", c)) ...

After the previous few commits reorganised the int-arg getting code, this
block can now be moved into an int-only section, so not slowing down
other format types.

There should be no functional changes.

I've added some comments to the %c branch explaining why its a special
case.