xor to make zero
xor to make zero

Just noticed this while digging through skia:10210.

Not super important, but does make programs a little smaller,
and the vpxor reg,reg,reg stands out clearly in disassembly.

Could do the same for all-ones, but I think that's mostly
used from memory directly, by vptest.

Change-Id: I8372f7071a3c951f879d440bbd32640ebdfabf08
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/287858
Commit-Queue: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Auto-Submit: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>

google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia