cmd/compile: enhance prove to deal with double-offset IsInBounds checks
For chunked iterations (useful for, but not exclusive to,
SIMD calculations) it is common to see the combination of
```
for ; i <= len(m)-4; i += 4 {
```
and
```
r0, r1, r2, r3 := m[i], m[i+1], m[i+2], m[i+3]
``
Prove did not handle the case of len-offset1 vs index+offset2
checking, but this change fixes this. There may be other
similar cases yet to handle -- this worked for the chunked
loops for simd, as well as a handful in std.
Cherry-picked from the dev.simd branch. This CL is not
necessarily SIMD specific. Apply early to reduce risk.
Change-Id: I3785df83028d517e5e5763206653b34b2befd3d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/700696 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/+/708859 Reviewed-by: David Chase <drchase@google.com>