cmd/compile: remove unused ISELB PPC64 ssa opcode

cmd/compile: remove unused ISELB PPC64 ssa opcode

The usage of ISELB has been removed as part of changes
made to support Power10 SETBC instructions.

Change-Id: I2fce4370f48c1eeee65d411dfd1bea4201f45b45
Reviewed-on: https://go-review.googlesource.com/c/go/+/465575
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Archana Ravindar <aravind5@in.ibm.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>