[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@237474 91177308-0d34-0410-b5e6-96231b3b80d8

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