test/codegen: tidy tests for bits

test/codegen: tidy tests for bits

Use Go idiomatic function names, use a common prefix, attempt to
maintain some consistency, avoid naming functions based upon
machine specific instructions and combine a duplicate test that
likely exists due to this confusion.

Change-Id: I996e9efd7497821edef94c1997d4a310d9d79a71
Reviewed-on: https://go-review.googlesource.com/c/go/+/732200
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Joel Sing <joel@sing.id.au>
Reviewed-by: Michael Knyszek <mknyszek@google.com>