cmd/objdump: MOVZX disassembly is ambiguous

[randall77]

Is it zeroing the top 56, 48, or 32 bits? Not clear from the disassembler.
Similarly for MOVSX - where is it sign-extending from?
In gcc syntax you can tell from the size of the register argument. Not so for plan9 syntax.

I fixed something similar for POPCNT in https://go-review.googlesource.com/c/arch/+/351889/ , but that strategy won't work here because the size used there is the output size, not the input size.