cmd/compile: enhance the chunked indexing case to include reslicing

cmd/compile: enhance the chunked indexing case to include reslicing

this helps SIMD, but also helps plain old Go

Cherry-picked from the dev.simd branch. This CL is not
necessarily SIMD specific. Apply early to reduce risk.

Change-Id: Idcdacd54b6776f5c32b497bc94485052611cfa8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/704756
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/708862
Reviewed-by: David Chase <drchase@google.com>