[dev.simd] simd, cmd/compile: implement ToMask, unexport asMask.

[dev.simd] simd, cmd/compile: implement ToMask, unexport asMask.

This CL defines the mask semantic better:
When converting from vector to mask, its element is set to true iff
the corresponding vector element is non zero.

Change-Id: I331c1c7992dc9e81c211bdc6d73e5eb3b8414506
Reviewed-on: https://go-review.googlesource.com/c/go/+/697056
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>