CellSPU:

- Ensure that (operation) legalization emits proper FDIV libcall when needed.
- Fix various bugs encountered during llvm-spu-gcc build, along with various
  cleanups.
- Start supporting double precision comparisons for remaining libgcc2 build.
  Discovered interesting DAGCombiner feature, which is currently solved via
  custom lowering (64-bit constants are not legal on CellSPU, but DAGCombiner
  insists on inserting one anyway.)
- Update README.

llvm-svn: 62664