fix dynamic stack allocs for amd64

The arm64 might have the same problem but it
is currently unable to handle them even in
instruction selection.

Thanks to Jean Dao for reporting the bug.

test/dynalloc.ssa

@@ -0,0 +1,27 @@
+# make sure dynamic allocations
+# and caller-save regs interact
+# soundly
+
+function $g() {
+@start
+	ret
+}
+
+function w $f(w %arg) {
+@start
+	call $g()
+@alloc
+	%r =l alloc8 16
+	storel 180388626474, %r
+	%r8 =l add 8, %r
+	storel 180388626474, %r8
+	ret %arg
+}
+
+export
+function w $main() {
+@start
+	%a =w call $f(w 0)
+	%b =w call $f(w 0)
+	ret %a
+}