x/tools/gopls/internal/analysis/modernize: incomplete simplification in appendclipped

[adonovan]

The appendclipped modernizer has an existing test case whose output (after applying a fix) is:

	print(slices.Concat(other[:len(other):len(other)], s, other))      // want "Replace append with slices.Concat"

The first argument should also have been simplified to other. Investigate + fix.

[gopherbot]

Change https://go.dev/cl/647796 mentions this issue: gopls/internal/analysis/modernize: appendclipped: unclip