[dev.simd] cmd/compile, simd: API interface fixes

[dev.simd] cmd/compile, simd: API interface fixes

    - Absolute -> Abs
    - ApproximateReciprocal -> Reciprocal
      - Other derived apis also changed.
    - Round -> RoundToEven
      - Other derived apis also changed.
    - Drop DotProdBroadcast
    - Fused(Mul|Add)(Mul|Add)? -> remove the "Fused"
    - MulEvenWiden -> remove 64bit
    - MulLow -> Mul, add unit
    - PairDotProd -> DotProdPairs
      - make AddDotProdPairs machine ops only - peepholes will be in another
        CL at dev.simd.
    - PopCount -> OnesCount
    - Saturated* -> *Saturated
    - Fix (Add|Sub)Saturated uint mappings.
    - UnsignedSignedQuadDotProdAccumulate -> AddDotProdQuadruple
      - The "DotProdQuadruple" instruction does not exist, so no peepholes for
        this.
This CL is generated by CL 694095.

Change-Id: If4110cc04ab96240cf56f2348d35ed2a719687de
Reviewed-on: https://go-review.googlesource.com/c/go/+/694115
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>