cmd/compile: remove duplicate ppc64 rules

Const64 gets lowered to MOVDconst.
Change rules using interior Const64 to use MOVDconst instead,
to be less dependent on rule application order.

As a result of doing this, some of the rules end up being
exact duplicates; remove those.

We had those exact duplicates because of the order dependency;
ppc64 had no way to optimize away shifts by a constant
if the initial lowering didn't catch it.

Add those optimizations as well.
The outcome is the same, but this makes the overall rules more robust.

Change-Id: Iadd97a9fe73d52358d571d022ace145e506d160b
Reviewed-on: https://go-review.googlesource.com/c/go/+/220877
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>

src/cmd/compile/internal/ssa/opGen.go

@@ -1881,6 +1881,8 @@ const (
 	OpPPC64FlagEQ
 	OpPPC64FlagLT
 	OpPPC64FlagGT
+	OpPPC64FlagCarrySet
+	OpPPC64FlagCarryClear
 
 	OpRISCV64ADD
 	OpRISCV64ADDI
@@ -24995,6 +24997,16 @@ var opcodeTable = [...]opInfo{
 		argLen: 0,
 		reg:    regInfo{},
 	},
+	{
+		name:   "FlagCarrySet",
+		argLen: 0,
+		reg:    regInfo{},
+	},
+	{
+		name:   "FlagCarryClear",
+		argLen: 0,
+		reg:    regInfo{},
+	},
 
 	{
 		name:        "ADD",