[SROA] Add an off-by-default *strict* inbounds check to SROA. I had SROA
implemented this way a long time ago and due to the overwhelming bugs
that surfaced, moved to a much more relaxed variant. Richard Smith would
like to understand the magnitude of this problem and it seems fairly
harmless to keep some flag-controlled logic to get the extremely strict
behavior here. I'll remove it if it doesn't prove useful.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202193 91177308-0d34-0410-b5e6-96231b3b80d8