cgen: default the hi bound of an N_DOT array-field slice arg

pushargsrev's N_SLICE arm resolved esz and the base address from the
stamped tinfo of an N_DOT [N]T-field base but let the omitted hi bound
fall to $0, so the #258 array-to-slice borrow desugar handed callees a
zero-length slice: base64 clear()'s bytes.zero(e.ibuf) was a runtime
no-op under the wwstage frontend. Take the element count from the
field's array tinfo like the cgslice #252 arm and cstage's bu->alen.
Graduates r989_libprecond_decodedsize_aligned and the 989_lib_byteid
#59.3 base64 pin.

selfhost/cmd/wcc/cgenutil.ww

@@ -774,9 +774,17 @@ fn pushargsrev(c: *cgen, arg: *syntax.node, param: *syntax.node, memphase: bool,
 					emitline("\tMOVQ\t8(CX), AX\n");
 				};};
 			};
+		} else { if (dotbu != nil && dotbu.kind == syntax.tykind.TY_ARRAY) {
+			// #252 twin: default-hi over an N_DOT `[N]T`-field base —
+			// element count from the field's array tinfo (cstage reads
+			// bu->alen uniformly). Without this the #258 array→slice
+			// borrow desugar handed callees a 0-length slice.
+			emitline("\tMOVQ\t$");
+			emitint(dotbu.alen: i64);
+			emitline(", AX\n");
 		} else {
 			emitline("\tMOVQ\t$0, AX\n");
-		};};};
+		};};};};
 		emitline("\tPUSHQ\tAX\n");
 		// lo (default 0) → AX
 		if (lo != nil) { cgexpr(c, lo); }