optimize Perl_boot_core_UNIVERSAL and Perl_nextargv

Perl_nextargv fuse 2 tests into 1

Perl_boot_core_UNIVERSAL reorder instructions to avoid using a non-vol
registers (save cv across the Safefree), not reread CV body * after
Safefree. Compute address of CvFILE once.

for Perl_boot_core_UNIVERSAL on VC 2003 before 0x66 bytes of 32 bit x86
machine code, after 0x61.