cmd/compile: mark abi.PtrType.Elem sym as used

CL 700336 let the compiler see into the abi.PtrType.Elem field,
but forgot the MarkTypeSymUsedInInterface to ensure that the symbol
is marked as referenced.

I am not sure how to write a test for this, but I noticed this when
working on further optimizations where I "fixed" this issue and
confusingly failed toolstash -cmp, with diffs like:

@@ -70582,6 +70582,7 @@ reflect.groupAndSlotOf<1> STEXT size=696 args=0x20 locals=0x1e0 funcid=0x0 align
 	rel 3+0 t=R_USEIFACE type:*reflect.rtype<0>+0
 	rel 3+0 t=R_USEIFACE type:*reflect.rtype<0>+0
 	rel 3+0 t=R_USEIFACE type:*uint64<0>+0
+	rel 3+0 t=R_USEIFACE type:uint64<0>+0
 	rel 71+0 t=R_CALLIND +0
 	rel 92+4 t=R_PCREL go:itab.*reflect.rtype,reflect.Type<0>+0
 	rel 114+4 t=R_CALL reflect.(*rtype).ptrTo<1>+0

Updates golang#75203

Change-Id: Ib8de8a32aeb8a7ea6fcf5d728a2e4944ef227ab2
Reviewed-on: https://go-review.googlesource.com/c/go/+/701296
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: jakebailey

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

@@ -2778,10 +2778,10 @@
 (Load <typ.Uintptr> (ITab (IMake (Convert (Addr {s} sb) _) _)) _) && isFixedSym(s, 0) => (Addr {fixedSym(b.Func, s, 0)} sb)
 
 // Loading constant values from abi.PtrType.Elem.
-(Load <t> (OffPtr [off]                       (Addr {s} sb)       ) _) && t.IsPtr() && isPtrElem(s, off) => (Addr {ptrElem(s, off)} sb)
-(Load <t> (OffPtr [off]              (Convert (Addr {s} sb) _)    ) _) && t.IsPtr() && isPtrElem(s, off) => (Addr {ptrElem(s, off)} sb)
-(Load <t> (OffPtr [off] (ITab (IMake          (Addr {s} sb)    _))) _) && t.IsPtr() && isPtrElem(s, off) => (Addr {ptrElem(s, off)} sb)
-(Load <t> (OffPtr [off] (ITab (IMake (Convert (Addr {s} sb) _) _))) _) && t.IsPtr() && isPtrElem(s, off) => (Addr {ptrElem(s, off)} sb)
+(Load <t> (OffPtr [off]                       (Addr {s} sb)       ) _) && t.IsPtr() && isPtrElem(s, off) => (Addr {ptrElem(b.Func, s, off)} sb)
+(Load <t> (OffPtr [off]              (Convert (Addr {s} sb) _)    ) _) && t.IsPtr() && isPtrElem(s, off) => (Addr {ptrElem(b.Func, s, off)} sb)
+(Load <t> (OffPtr [off] (ITab (IMake          (Addr {s} sb)    _))) _) && t.IsPtr() && isPtrElem(s, off) => (Addr {ptrElem(b.Func, s, off)} sb)
+(Load <t> (OffPtr [off] (ITab (IMake (Convert (Addr {s} sb) _) _))) _) && t.IsPtr() && isPtrElem(s, off) => (Addr {ptrElem(b.Func, s, off)} sb)
 
 // Loading constant values from runtime._type.hash.
 (Load <t> (OffPtr [off]                       (Addr {sym} _)       ) _) && t.IsInteger() && t.Size() == 4 && isFixed32(config, sym, off) => (Const32 [fixed32(config, sym, off)])

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

@@ -2029,14 +2029,16 @@ func isPtrElem(sym Sym, off int64) bool {
 	}
 	return false
 }
-func ptrElem(sym Sym, off int64) Sym {
+func ptrElem(f *Func, sym Sym, off int64) Sym {
 	lsym := sym.(*obj.LSym)
 	if strings.HasPrefix(lsym.Name, "type:*") {
 		if ti, ok := (*lsym.Extra).(*obj.TypeInfo); ok {
 			t := ti.Type.(*types.Type)
 			if t.Kind() == types.TPTR {
 				if off == rttype.PtrType.OffsetOf("Elem") {
-					return reflectdata.TypeLinksym(t.Elem())
+					elemSym := reflectdata.TypeLinksym(t.Elem())
+					reflectdata.MarkTypeSymUsedInInterface(elemSym, f.fe.Func().Linksym())
+					return elemSym
 				}
 			}
 		}