This is intended to allow IDEs to note where the optimizer
was not able to improve users' code.  There may be other
applications for this, for example in studying effectiveness
of optimizer changes more quickly than running benchmarks,
or in verifying that code changes did not accidentally disable
optimizations in performance-critical code.

Logging of nilcheck (bad) for amd64 is implemented as
proof-of-concept.  In general, the intent is that optimizations
that didn't happen are what will be logged, because that is
believed to be what IDE users want.

Added flag -json=version,dest

Check that version=0.  (Future compilers will support a
few recent versions, I hope that version is always <=3.)

Dest is expected to be one of:

/path (or \path in Windows)
  will create directory /path and fill it w/ json files
file://path
  will create directory path, intended either for
     I:\dont\know\enough\about\windows\paths
     trustme_I_know_what_I_am_doing_probably_testing

Not passing an absolute path name usually leads to
json splattered all over source directories,
or failure when those directories are not writeable.
If you want a foot-gun, you have to ask for it.

The JSON output is directed to subdirectories of dest,
where each subdirectory is net/url.PathEscape of the
package name, and each for each foo.go in the package,
net/url.PathEscape(foo).json is created.  The first line
of foo.json contains version and context information,
and subsequent lines contains LSP-conforming JSON
describing the missing optimizations.

Change-Id: Ib83176a53a8c177ee9081aefc5ae05604ccad8a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/204338
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>