[LICM] Store sink and indirectbr instructions

Loop simplify skips exit-block insertion when exits contain indirectbr
instructions. This leads to an assertion in LICM when trying to sink
stores out of non-dedicated loop exits containing indirectbr
instructions. This patch fix this issue by re-checking for dedicated
exits in LICM prior to store sink attempts.

Differential Revision: http://reviews.llvm.org/D6414

rdar://problem/18943047

llvm-svn: 222927