cmd/internal/obj/ppc64: cleanup optab entries after renaming

cmd/internal/obj/ppc64: cleanup optab entries after renaming

Combine MOVW/MOVD using C_16CON as they accept any 16 bit
constant.

Remove MULLW/MULLD C_U16CON optab entry. These assemble to
the mulli opcode which only accepts a signed 16 bit constant.

Remove superfluous optab entrys for VSPLTB and VSPLTISB,
as C_S16CON accepts C_U15CON arguments.

Change-Id: Ie20dd07bcedda428fb1dd674474d7dfa67d76dc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/563915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Eli Bendersky <eliben@google.com>