cmd/compile: use unsigned constant when folding loads for SIMD ops with constants

cmd/compile: use unsigned constant when folding loads for SIMD ops with constants

When folding loads into a SIMD op with a constant, in the SSA
rules we use makeValAndOff to create an AuxInt for the constant
and the offset. For the SIMD ops of concern (for now), the
constants are always unsigned. So pass the constant unsigned.

Fixes #76756.

Change-Id: Ia5910e689ff510ce54d3a0c2ed0e950bc54f8862
Reviewed-on: https://go-review.googlesource.com/c/go/+/730420
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>