internal/lsp: return early in completion where possible
internal/lsp: return early in completion where possible

This a pure cut-and-paste.

Fixes golang/go#38868

Change-Id: I2ff07134a8b9f6186bab737caceab8a34a8e2f44
Reviewed-on: https://go-review.googlesource.com/c/tools/+/232677
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools