Reapply r233175 and r233183: float2int.

This re-adds float2int to the tree, after fixing PR23038. It turns
out the argument to APSInt() is true-if-unsigned, rather than
true-if-signed :(. Added testcase and explanatory comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233370 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Transforms/Scalar.h

@@ -446,6 +446,12 @@ ModulePass *createPlaceSafepointsPass();
 //
 FunctionPass *createRewriteStatepointsForGCPass();
 
+//===----------------------------------------------------------------------===//
+//
+// Float2Int - Demote floats to ints where possible.
+//
+FunctionPass *createFloat2IntPass();
+
 } // End llvm namespace
 
 #endif