cmd/compile: fix typing of IData opcodes

The rules for extracting the interface data word don't leave
the result typed correctly. If I do i.([1]*int)[0], the result
should have type *int, not [1]*int. Using (IData x) for the result
keeps the typing of the original top-level Value.

I don't think this would ever cause a real codegen bug, bug fixing it
at least makes the typing shown in ssa.html more consistent.

Change-Id: I239d821c394e58347639387981b0510d13b2f7b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/204042
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -803,7 +803,7 @@
 
 // Putting struct{*byte} and similar into direct interfaces.
 (IMake typ (StructMake1 val)) -> (IMake typ val)
-(StructSelect [0] x:(IData _)) -> x
+(StructSelect [0] (IData x)) -> (IData x)
 
 // un-SSAable values use mem->mem copies
 (Store {t} dst (Load src mem) mem) && !fe.CanSSA(t.(*types.Type)) ->
@@ -823,9 +823,9 @@
 (Store _ (ArrayMake0) mem) -> mem
 (Store dst (ArrayMake1 e) mem) -> (Store {e.Type} dst e mem)
 
-// Putting [1]{*byte} and similar into direct interfaces.
+// Putting [1]*byte and similar into direct interfaces.
 (IMake typ (ArrayMake1 val)) -> (IMake typ val)
-(ArraySelect [0] x:(IData _)) -> x
+(ArraySelect [0] (IData x)) -> (IData x)
 
 // string ops
 // Decomposing StringMake and lowering of StringPtr and StringLen

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

@@ -6142,18 +6142,18 @@ func rewriteValuegeneric_OpArraySelect_0(v *Value) bool {
 		v.AddArg(x)
 		return true
 	}
-	// match: (ArraySelect [0] x:(IData _))
-	// result: x
+	// match: (ArraySelect [0] (IData x))
+	// result: (IData x)
 	for {
 		if v.AuxInt != 0 {
 			break
 		}
-		x := v.Args[0]
-		if x.Op != OpIData {
+		v_0 := v.Args[0]
+		if v_0.Op != OpIData {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpIData)
 		v.AddArg(x)
 		return true
 	}
@@ -43502,18 +43502,18 @@ func rewriteValuegeneric_OpStructSelect_10(v *Value) bool {
 		v0.AddArg(mem)
 		return true
 	}
-	// match: (StructSelect [0] x:(IData _))
-	// result: x
+	// match: (StructSelect [0] (IData x))
+	// result: (IData x)
 	for {
 		if v.AuxInt != 0 {
 			break
 		}
-		x := v.Args[0]
-		if x.Op != OpIData {
+		v_0 := v.Args[0]
+		if v_0.Op != OpIData {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpIData)
 		v.AddArg(x)
 		return true
 	}