[AMDGPU] Build lane intrinsics in a mangling-agnostic way. NFC. (#91583)

Use the form of CreateIntrinsic that takes an explicit return type and
works out the mangling based on that and the types of the arguments. The
advantage is that this still works if intrinsics are changed to have
type mangling, e.g. if readlane/readfirstlane/writelane are changed to
work on any type.