[perl #42349] [TODO] dotnet - fix various ops for 64 bit architectures

[Paul Cochrane]

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In languages/dotnet/ops/dotnet.ops, the following ops need to be fixed
for 64 bit architectures:

- net_conv_ovf_i_un
- net_conv_ovf_u_un
- net_conv_ovf_i
- net_conv_ovf_u