[PATCH][Interpreter] Add missing FP intrinsic lowering.

Summary:
This extends the work done in [1], adding missing intrinsic lowering for floor, trunc, round and copysign.

[1] http://comments.gmane.org/gmane.comp.compilers.llvm.cvs/199372

Test Plan: Extended `test/ExecutionEngine/Interpreter/intrinsics.ll` to test the additional missing intrinsics. All tests pass.

Reviewers: dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5120