remove duplicate SvNV calls in pp_enteriter

commit a2309040b8 added duplicate SvNV calls, remove them. Reorder the
"SvUV_nomg(sv) > (UV)IV_MAX || SvNV_nomg(sv) > (NV)UV_MAX" so the var
will be stored in a FP CPU reg for all comparisons, and not saved/fetched
to/from mem across the SvUV func call. Due to complexity, I am not
unrolling and fusing SvNV_nomg and SvOK.

VC 2003 32b size of func in machine code bytes before 0x4d3 after 0x4a2
Update perldiag to reflect ‘Global symbol’ change

Why didn’t t/porting/diag.t catch this?