Avoid an unnecessary MOVL/MOVQ.

Follow CL 97377

Change-Id: Ic43976d6b0cece3ed455496d18aedd67e0337d3f
Reviewed-on: https://go-review.googlesource.com/97358
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>