Rename truncations to avoid colon in name

[tlively]

Having colons in instruction names would require special handling in
existing assemblers, which seems like unnecessary extra work.

[sunfishcode]

These names correspond to the names added in https://github.com/WebAssembly/nontrapping-float-to-int-conversions/. LLVM already uses these names. As it happens, those names are not actually final yet, and are under discussion in WebAssembly/nontrapping-float-to-int-conversions#4. I'm not attached to the ':' character, however I would like to see if we can find a logical naming scheme for these names, as this will likely be a pattern that many future opcodes will follow.

[tlively]

Thanks, @sunfishcode. I wasn't aware of the names in the nontrapping conversions proposal, so I will go chime in over there.

[tlively]

Closing this for now, since it is outdated and the discussion is happening elsewhere.