[dev.simd] cmd/compile, simd: complete AVX2? u?int shuffles

[dev.simd] cmd/compile, simd: complete AVX2? u?int shuffles

The namings follow the following convention:
- If its indices are from constant, amend "Constant" to the name.
- If its indices are used by multiple groups, mend "Grouped" to the
  name.
- If its indexing only the low part, amend "Lo", similarly "Hi".

Change-Id: I6a58f5dae54c882ebd59f39b5288f6f3f14d957f
Reviewed-on: https://go-review.googlesource.com/c/go/+/698296
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>