[dev.simd] cmd/compile: flip sense of intrinsics test for SIMD

[dev.simd] cmd/compile: flip sense of intrinsics test for SIMD

ENABLE when simd experiment is off, to be sure intrinsics
do not leak past the experiment.

DISABLE when simd is on, because all this does is cause tests to
fail, then whoever failed the test regenerates the simd, doesn't look
at the mountain of new intrinsics, and just rubber-stamps the change.
All friction, no benefit.

Change-Id: I2ef7e0c246aaddd4a52c1d6108cb587adc1b8366
Reviewed-on: https://go-review.googlesource.com/c/go/+/677555
Auto-Submit: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>