[dev.simd] simd: rename LoLong intrinsics to WidenLo

MulLoLong → MulWidenLo, ShiftLeftLoLongConst → ShiftLeftWidenLoConst.
Consistent with wasm's MulWiden{Lo,Hi} naming convention.

Change-Id: I58c4624f62ab977ca37822390779d13019a0c37d
Reviewed-on: https://go-review.googlesource.com/c/go/+/784120
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>