simd: for 1.27, make ARM64 Reduce* methods return scalar

This is a cherry pick of Alexander Musman's CL 784800,
done this way to help with 1.27 release deadlines and
a minor amount of merging friction.

ReduceSum/ReduceMax/ReduceMin on all types now return scalar (e.g. int32)
instead of vector. Internally, the intrinsics are unexported (lowercase)
and return a vector with the result in lane 0. The exported wrappers
call GetElem(0) to extract the scalar, generated by tmplgen.

A follow-up CL will add peephole rules to elide the GetElem(0)
when the result feeds directly into another SIMD operation.

Former-Change-Id: I630cec6a26d54ed9ed1412a37076e20df9e51038
Change-Id: Id8ff6e07083bc62de4ca595dea3c9884b75ec517
Reviewed-on: https://go-review.googlesource.com/c/go/+/787560
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
14 files changed
tree: 9344f7fad79260177089e6f32a28154d58ba588f
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. codereview.cfg
  11. CONTRIBUTING.md
  12. go.env
  13. LICENSE
  14. PATENTS
  15. README.md
  16. SECURITY.md
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