CellSPU:

- Fix fabs, fneg for f32 and f64.
- Use BuildVectorSDNode.isConstantSplat, now that the functionality exists
- Continue to improve i64 constant lowering. Lower certain special constants
  to the constant pool when they correspond to SPU's shufb instruction's
  special mask values. This avoids the overhead of performing a shuffle on a
  zero-filled vector just to get the special constant when the memory load
  suffices.

llvm-svn: 67067