Add an "are types equivalent" operation that ignores the types that a pointer

Add an "are types equivalent" operation that ignores the types that a pointer
points to while analyzing all other fields.

Use FoldingSetNodeID to produce a good hash. This dramatically decreases run
times.

Emit thunks. This means that it can look at all functions regardless of what
the linkage is or if the address is taken, but unfortunately some small
functions can be even shorter than the thunk because our backend doesn't yet
realize it can just turn these into jumps. This means that this pass will
pessimize code on average.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73222 91177308-0d34-0410-b5e6-96231b3b80d8