BoundsChecking: optimize out the check for offset < 0 if size is known to be >=...

BoundsChecking: optimize out the check for offset < 0  if size is known to be >= 0 (signed).
(LLVM optimizers cannot do this optimization by themselves)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159668 91177308-0d34-0410-b5e6-96231b3b80d8