Print GEP offsets as signed values instead of unsigned values. On X86, this

Print GEP offsets as signed values instead of unsigned values.  On X86, this
prints:

getelementptr (int* %A, int -1)

as: "(A) - 4" instead of "(A) + 18446744073709551612", which makes the
assembler much happier.

This fixes test/Regression/CodeGen/X86/2005-02-14-IllegalAssembler.ll,
and Benchmarks/Prolangs-C/cdecl with LLC on X86.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20183 91177308-0d34-0410-b5e6-96231b3b80d8