Run SimplifyLibCalls near the beginning, not at

Run SimplifyLibCalls near the beginning, not at
the end.  It is now run at the same moment as in
llvm-gcc.  Also, run StripDeadPrototypes at the
end, just before running DeadTypeElimination.
This may be useful when doing LTO.  Note that
llvm-gcc runs StripDeadPrototypes but not
DeadTypeElimination.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49848 91177308-0d34-0410-b5e6-96231b3b80d8