cmd/gofmt: use SkipObjectResolution with -s as well

The "simplify" feature used go/ast's object tracking in only one place -
to replace s[a:len(s)] with s[a:].
Using go/ast.Object did allow us to not simplify code like:

	len := func(s []int) int { ... }
	s = s[a:len(s)]

The existing code already noted the limitation with that approach,
such as "len" being redeclared in a different file in the same package.
Since go/ast's object tracking is file-based and very basic,
it wouldn't work with edge cases like those.

The reasoning is that redeclaring len and abusing it that way is
extremely unlikely, and hasn't been a problem in about a decade now.
I reason that the same applies to len being redeclared in the same file,
so we should be able to safely remove the use of go/ast.Object here.

Per https://go.dev/cl/401454, this makes "gofmt -s" about 5% faster.
If we ever wanted to truly get rid of false positive simplifications,
I imagine we'd want to reimplement the feature under go/analysis,
which is able to fully typecheck packages and suggest edits.
That seems unnecessary at this point, but we can always course correct
in the presumably unlikely scenario that users start reporting bugs.

See #46485.
For #52463.

Change-Id: I77fc97adceafde8f0fe6887ace83ae325bfa7416
Reviewed-on: https://go-review.googlesource.com/c/go/+/401875
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

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