This is a partial fix for PR24886:
Without this IR transform, the backend (x86 at least) was producing inefficient code.
This patch is making 2 assumptions:
- The canonical form of a fabs() operation is, in fact, the LLVM fabs() intrinsic.
- The high bit of an FP value is always the sign bit; as noted in the bug report, this isn't specified by the LangRef.