cmd/compile: make loopbce handle 8, 16 and 32 bit induction variables

cmd/compile: make loopbce handle 8, 16 and 32 bit induction variables

Compute limits and increment values for all integer widths.
Resolves 2 TODO's in loopbce.go

compilecmp linux/amd64:

compress/flate
compress/flate.(*huffmanEncoder).bitCounts 1235 -> 1207  (-2.27%)

cmd/internal/obj/wasm
cmd/internal/obj/wasm.assemble 7443 -> 7303  (-1.88%)
cmd/internal/obj/wasm.assemble.func1 165 -> 138  (-16.36%)

cmd/link/internal/ld
cmd/link/internal/ld.(*Link).findfunctab.func1 1646 -> 1627  (-1.15%)

Change-Id: I2d79b7376eb67d6bcc8fdaf0c197c11e631562d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/435258
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>