cmd/compile: allow load-op merging in additional situations
x += *p
We want to do this with a single load+add operation on amd64.
The tricky part is that we don't want to combine if there are
other uses of x after this instruction.
Implement a simple detector that seems to capture a common situation -
x += *p is in a loop, and the other use of x is after loop exit.
In that case, it does not hurt to do the load+add combo.
Change-Id: I466174cce212e78bde83f908cc1f2752b560c49c
Reviewed-on: https://go-review.googlesource.com/c/go/+/672957 Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>