cmd/compile: simplify intrinsification of BitLen16 and BitLen8

cmd/compile: simplify intrinsification of BitLen16 and BitLen8

Decompose BitLen16 and BitLen8 within the SSA rules for architectures that
support BitLen32 or BitLen64, rather than having a custom intrinsic.

Change-Id: Ie4188ce69d1021e63cec27a8e7418efb0714812b
Reviewed-on: https://go-review.googlesource.com/c/go/+/651817
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Michael Knyszek <mknyszek@google.com>