vector: tweak the asm register assignment.

This makes the fixed point and floating point code more similar.

Benchmarks don't show any significant change.

Change-Id: I723fa1605eaa248b40e784201b680c16cc3d26a2
Reviewed-on: https://go-review.googlesource.com/32134
Reviewed-by: David Crawshaw <crawshaw@golang.org>
2 files changed