cmd/compile: support float min/max instructions on PPC64

cmd/compile: support float min/max instructions on PPC64

This enables efficient use of the builtin min/max function
for float64 and float32 types on GOPPC64 >= power9.

Extend the assembler to support xsminjdp/xsmaxjdp and use
them to implement float min/max.

Simplify the VSX xx3 opcode rules to allow FPR arguments,
if all arguments are an FPR.

Change-Id: I15882a4ce5dc46eba71d683cf1d184dc4236a328
Reviewed-on: https://go-review.googlesource.com/c/go/+/574535
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Than McIntosh <thanm@google.com>