cmd/compile: fix bit length intrinsic for 16/8 bits on GOAMD64=v3

cmd/compile: fix bit length intrinsic for 16/8 bits on GOAMD64=v3

Upper bits of registers for uint8/uint16 are junk. Make sure we
mask those off before using LZCNT (leading zeros count).

Fixes #52681

Change-Id: I0ca9e62f23bcb1f6ad2a787fa9895322afaa2533
Reviewed-on: https://go-review.googlesource.com/c/go/+/403815
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@google.com>