[dev.simd] simd: move lots of slice functions and methods to generated code

[dev.simd] simd: move lots of slice functions and methods to generated code

Lots of handwritten/stenciled code is now untouched by human hands

For certain combinations of operation-arity and type, there
is an option to use a flaky version of a test helper, that only
requires "close enough".  For example:

testFloat32x4TernaryFlaky(t, simd.Float32x4.FusedMultiplyAdd, fmaSlice[float32], 0.001)

Some of the quirkier operations have their behavior captured
in their test-simulation, for example, ceilResidue regards
infinities as integers (therefore their residue is zero).

Change-Id: I8242914e5ab399edbe226da8586988441cffa83f
Reviewed-on: https://go-review.googlesource.com/c/go/+/690575
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>