encoding/xml: rewrite func procInst
This CL tries to make function procInst more exact,
also adds test cases, however, including tricky ones.
Change-Id: If421299fc84d136e56a25dba7a4919c4424702c8
GitHub-Last-Rev:
b9a3192718ae5535d66fddd260c27b48d93b4af1
GitHub-Pull-Request: golang/go#64336
Reviewed-on: https://go-review.googlesource.com/c/go/+/544475
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>