[dev.simd] cmd/compile: use X15 for zero value in AVX context

[dev.simd] cmd/compile: use X15 for zero value in AVX context

With the previous CL, the X15 (aliasd with Y15, Z15) register
holds the zero value for the whole register width. Use that in
AVX context when a zero value is needed.

Change-Id: If49b7059bce50c5e86f90bace0eaa830a91fa0fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/698238
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
TryBot-Bypass: Cherry Mui <cherryyz@google.com>