LSR fix: Add isSimplifiedLoopNest to IVUsers analysis.

Only record IVUsers that are dominated by simplified loop
headers. Otherwise SCEVExpander will crash while looking for a
preheader.

I previously tried to work around this in LSR itself, but that was
insufficient. This way, LSR can continue to run if some uses are not
in simple loops, as long as we don't attempt to analyze those users.

Fixes <rdar://problem/11049788> Segmentation fault: 11 in LoopStrengthReduce

llvm-svn: 152892