Fix PR5471 by removing an instcombine xform. Some pieces of the code

Fix PR5471 by removing an instcombine xform.  Some pieces of the code
generates store to undef and some generates store to null as the idiom
for undefined behavior.  Since simplifycfg zaps both, don't remove the
undefined behavior in instcombine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89971 91177308-0d34-0410-b5e6-96231b3b80d8