Document assumption in X86FrameLowering::inlineStackProbe()
Document assumption in X86FrameLowering::inlineStackProbe()

Resolve FIXME from r261504.  Apparently bundled instructions are illegal
here:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160215/334146.html

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@261507 91177308-0d34-0410-b5e6-96231b3b80d8

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