[X86] Use a better sentinel offset for the FrameAddr index

[X86] Use a better sentinel offset for the FrameAddr index

Other pieces of CodeGen want to negate frame object offsets to account
for architectures where the stack grows down.  Our object is a pseudo
object so it's offset doesn't matter.  However, we shouldn't choose an
offset which results in undefined behavior if you negate it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237474 91177308-0d34-0410-b5e6-96231b3b80d8