simd/archsimd: remove redundant suffix of ExtendLo operations

simd/archsimd: remove redundant suffix of ExtendLo operations

For methods like ExtendLo2ToInt64x2, the last "x2" is redundant, as
it is already mentioned in "Lo2". Remove it, so it is just
ExtendLo2ToInt64.

Change-Id: I490afd818c40bb7a4ef15c249723895735bd6488
Reviewed-on: https://go-review.googlesource.com/c/go/+/733100
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>