decodetree: Properly diagnose fields overflowing an insn

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

Backports commit 2decfc95583dc28add69810eaca6ada7b4b44d3a from qemu

qemu/scripts/decodetree.py

@@ -756,6 +756,8 @@ def parse_generic(lineno, is_format, name, toks):
                 sign = True
                 flen = flen[1:]
             shift = int(flen, 10)
+            if shift + width > insnwidth:
+                error(lineno, 'field {0} exceeds insnwidth'.format(fname))
             f = Field(sign, insnwidth - width - shift, shift)
             flds = add_field(lineno, flds, fname, f)
             fixedbits <<= shift