Don't generate llvm.expect intrinsics with -O0.

The backend won't run LowerExpect on -O0.  In a debug LTO build, this results in llvm.expect intrinsics being in the LTO IR which doesn't know how to optimize them.

Thanks to Chandler for the suggestion and review.

Differential revision: http://reviews.llvm.org/D7183

llvm-svn: 227135