[dev.simd] cmd/compile, simd: change SHA ops names and types

[dev.simd] cmd/compile, simd: change SHA ops names and types

This CL addressed some naming changes decided in API audit.
Before		After
SHA1Msg1	SHA1Message1, Remove signed
SHA1Msg2	SHA1Message2, Remove signed
SHA1NextE	SHA1NextE, Remove signed
SHA1Round4	SHA1FourRounds, Remove signed
SHA256Msg1	SHA256Message1, Remove signed
SHA256Msg2	SHA256Message2, Remove signed
SHA256Rounds2	SHA256TwoRounds, Remove signed

Change-Id: If2cead113f37a9044bc5c65e78fa9d124e318005
Reviewed-on: https://go-review.googlesource.com/c/go/+/721003
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>