cmd/internal/obj: remove bogus load/store optab entries from ppc64

No valid operation should match those removed by this patch. They
kind of look as if they match X-form load/stores on ppc64, but the
second argument is always ignored when translating to machine code.

Similarly, it should be noted an X-form memory access encodes into
an Addr which is a classified as a ZOREG argument with a non-zero
index, and a register type Addr.

Change-Id: I1adbb020d1b2612b18949d0e7eda05dbb3e8a25c
Reviewed-on: https://go-review.googlesource.com/c/go/+/303329


Reviewed-by: Carlos Eduardo Seo <carlos.seo@linaro.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Carlos Eduardo Seo <carlos.seo@linaro.org>