std::string to StringRef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176166 91177308-0d34-0410-b5e6-96231b3b80d8
IRBuilder has grown all sorts of useful utility functions. Make use of them to
clean up this code a tiny bit. No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176168 91177308-0d34-0410-b5e6-96231b3b80d8
For each function that we optimize we initialize a new list of lib functions. For each function name we malloc memory. This patch changes the Libcall map to use BumpPtrAllocator. Now we malloc only once. This speeds up instcombine by a few % on a large c++ program.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176170 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r176166 because it broke one of the lit tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176171 91177308-0d34-0410-b5e6-96231b3b80d8