cmd/asm: fix rlwnm reg,reg,const,reg encoding on ppc64

cmd/asm: fix rlwnm reg,reg,const,reg encoding on ppc64

The wrong value for the first reg parameter was selected.
Likewise the wrong opcode was selected.  This should match
rlwnm (rrr type), not rlwinm (irr type).

Similarly, fix the optab matching rules so clrlslwi does
not match reg,reg,const,reg arguments.  This is not a valid
operand combination for clrlslwi.

Fixes #42368

Change-Id: I4eb16d45a760b9fd3f497ef9863f82465351d39f
Reviewed-on: https://go-review.googlesource.com/c/go/+/267421
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>