Fix TableGen -gen-disassembler output for bit fields with an offset.

This fixes bit assignments like this
Inst{7-0} = Foo{9-2}

Patch by Steve King.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218560 91177308-0d34-0410-b5e6-96231b3b80d8