[SimplifyLibCalls] Fix instruction misplacement in string/memory libcall optimization

When trying to optimize fortified library functions use the right
location to insert new instructions in order to preserve correct
def-use order.

This fixes an issue where a misplaced instruction definition would
happen to be *after* one of its use after a RAUW, forming invalid IR.
This behavior was introduced by r227250.

Differential Revision: http://reviews.llvm.org/D13301

rdar://problem/22802369

llvm-svn: 249092