decodetree: Properly diagnose fields overflowing an insn

Previously this would result in an exception for shifting
the field mask by a negative number.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tests/decode/err_width4.decode

@@ -0,0 +1,5 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose too few bits (31 of 32)
+one	0 s:30