[dev.simd] cmd/compile: fix the "always panic" code to actually panic

[dev.simd] cmd/compile: fix the "always panic" code to actually panic

without this change, the intrinsics of non-constant immediates
just substitute a zero, which is wrong.

Change-Id: I2c39ebedcfb0d0d6c072f4434f393027c6f3f033
Reviewed-on: https://go-review.googlesource.com/c/go/+/685575
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>