[release-branch.go1.15] cmd/internal/obj/ppc64: don't remove NOP in assembler

Previously, the assembler removed NOPs from the Prog list in
obj9.go. NOPs shouldn't be removed if they were added as
an inline mark, as described in the issue below.

Fixes #40767

Once the NOPs were left in the Prog list, some instructions
were flagged as invalid because they had an operand which was
not represented in optab. In order to preserve the previous
assembler behavior, entries were added to optab for those
operand cases. They were not flagged as errors before because
the NOP instructions were removed before the code to check the
valid opcode/operand combinations.

Change-Id: Iae5145f94459027cf458e914d7c5d6089807ccf8
Reviewed-on: https://go-review.googlesource.com/c/go/+/247842


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Paul Murphy <murp@ibm.com>
Reviewed-by: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit 7d7bd5ab)
Reviewed-on: https://go-review.googlesource.com/c/go/+/248381
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>