[dev.ssa] cmd/compile: use function calls instead of special ops for panics

[dev.ssa] cmd/compile: use function calls instead of special ops for panics

There's no need for special ops for panicindex and panicslice.
Just use regular runtime calls.

Change-Id: I71b9b73f4f1ebce1220fdc1e7b7f65cfcf4b7bae
Reviewed-on: https://go-review.googlesource.com/14726
Reviewed-by: David Chase <drchase@google.com>