Pull some code out into a helper function.

Pull some code out into a helper function.
Effeciently codegen even splats in the range [-32,30].

This allows us to codegen <30,30,30,30> as:

        vspltisw v0, 15
        vadduwm v2, v0, v0

instead of as a cp load.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27750 91177308-0d34-0410-b5e6-96231b3b80d8