[LICM] Don't try to sink values out of loops without any exits

There is no suitable basic block to sink instructions in loops without
exits.  The only way an instruction in a loop without exits can be used
is as an incoming value to a PHI.  In such cases, the incoming block for
the corresponding value is unreachable.

This fixes PR24013.

Differential Revision: http://reviews.llvm.org/D10903

llvm-svn: 241987