go/parser: avoid formatting a panic message if an assertion succeeds
go/parser: avoid formatting a panic message if an assertion succeeds

tryResolve is an extremely hot method on the parser. Eliminating this
formatting led to a 20% performance improvement in BenchmarkParse.

Change-Id: Idf8850404bd72d45d1351356427a85086422ea68
Reviewed-on: https://go-review.googlesource.com/c/go/+/302629
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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