Relax validation of SCATTER/GATHER memory ops to spec, Fixes #193

[vsivsi]

See #193 and discussion here: #199 (comment)

[codecov-commenter]

Codecov Report

Merging #233 (42874d6) into master (fa3cfb0) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
- Coverage   75.95%   75.95%   -0.01%     
==========================================
  Files          65       65              
  Lines       20633    20694      +61     
==========================================
+ Hits        15672    15718      +46     
- Misses       4879     4894      +15     
  Partials       82       82              

Flag	Coverage Δ
integration	12.16% <50.00%> (+0.20%)	⬆️
unittests	73.01% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
operand/checks.go	99.21% <100.00%> (ø)
pass/verify.go	100.00% <100.00%> (ø)
internal/inst/types.go	52.63% <0.00%> (-3.71%)	⬇️
x86/zctors.go	100.00% <0.00%> (ø)
x86/zoptab.go	92.42% <0.00%> (ø)
internal/load/tables.go	100.00% <0.00%> (ø)
build/zinstructions.go	67.72% <0.00%> (+0.04%)	⬆️
internal/load/load.go	95.67% <0.00%> (+0.16%)	⬆️

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[vsivsi]

@mmcloughlin Happy to see recent activity on Avo. I'd love to get some PRs merged so I can get off my fork.

This one is a pretty straightforward fix, would be great to land it soon.

Not sure what's up with the pr/automerge fail above, looks like maybe I don't have some permission?