loop-reduce: Add an early bailout to catch extremely large loops.

This introduces a threshold of 200 IV Users, which is very
conservative but should be sufficient to avoid serious compile time
sink or stack overflow. The llvm test-suite with LTO never exceeds 190
users per loop.

The bug doesn't relate to a specific type of loop. Checking in an
arbitrary giant loop as a unit test would be silly.

Fixes rdar://11262507.

llvm-svn: 154983