cmd/compile: remove PDISCARD class

This pseudo-Class was previously used by the importer code to optimize
processing duplicate inline bodies, since we didn't actually care
about the declarations in those contexts.

This functionality is no longer needed with indexed exports, since we
now only process function bodies as needed (and never more than once).

Passes toolstash-check.

Change-Id: I7eab0cb16925ab777079c2a20731dbcfd63cf195
Reviewed-on: https://go-review.googlesource.com/c/go/+/200899Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/class_string.go

@@ -15,12 +15,11 @@ func _() {
 	_ = x[PPARAM-4]
 	_ = x[PPARAMOUT-5]
 	_ = x[PFUNC-6]
-	_ = x[PDISCARD-7]
 }
 
-const _Class_name = "PxxxPEXTERNPAUTOPAUTOHEAPPPARAMPPARAMOUTPFUNCPDISCARD"
+const _Class_name = "PxxxPEXTERNPAUTOPAUTOHEAPPPARAMPPARAMOUTPFUNC"
 
-var _Class_index = [...]uint8{0, 4, 11, 16, 25, 31, 40, 45, 53}
+var _Class_index = [...]uint8{0, 4, 11, 16, 25, 31, 40, 45}
 
 func (i Class) String() string {
 	if i >= Class(len(_Class_index)-1) {

src/cmd/compile/internal/gc/dcl.go

@@ -60,10 +60,6 @@ var declare_typegen int
 // declare records that Node n declares symbol n.Sym in the specified
 // declaration context.
 func declare(n *Node, ctxt Class) {
-	if ctxt == PDISCARD {
-		return
-	}
-
 	if n.isBlank() {
 		return
 	}

src/cmd/compile/internal/gc/go.go

@@ -60,17 +60,10 @@ const (
 	PPARAMOUT              // output results
 	PFUNC                  // global function
 
-	PDISCARD // discard during parse of duplicate import
 	// Careful: Class is stored in three bits in Node.flags.
-	// Adding a new Class will overflow that.
+	_ = uint((1 << 3) - iota) // static assert for iota <= (1 << 3)
 )
 
-func init() {
-	if PDISCARD != 7 {
-		panic("PDISCARD changed; does all Class values still fit in three bits?")
-	}
-}
-
 // note this is the runtime representation
 // of the compilers arrays.
 //