POWER9 (ISA 3.0) introduced a new format of load/store instructions to
implement indexed load/store quadword, using an immediate value instead
of a register index.

This change adds support for this new instruction encoding and adds the
new load/store quadword instructions (lxv/stxv) to the assembler.

This change also adds the missing XX1-form loads/stores (halfword and byte)
included in ISA 3.0.

Change-Id: Ibcdf53c342d7a352d64a9403c2fe7b25be9c3b24
Reviewed-on: https://go-review.googlesource.com/c/go/+/200399
Run-TryBot: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>