Codegen support for vector intrinsics.

Factor out the code that expands the "nasty scalar code" for unrolling
vectors into a separate routine, teach it how to handle mixed
vector/scalar operands, as seen in powi, and use it for several operators,
including sin, cos, powi, and pow.

Add support in SplitVectorOp for fpow, fpowi and for several unary
operators.

llvm-svn: 42884