[PowerPC] Split tests for constrained intrinsics

The test CodeGen/PowerPC/vector-constrained-fp-intrinsics.ll checks code
generation for constrained floating point intrinsics. Many test cases in
it were implemented using operations on constants. Constant folding of
constrained intrinsics would make these test cases almost useless,
because they would check only constant loading.

To keep the tests useful, operations on constants were replaced with
operations on function parameters.

Differential Revision: https://reviews.llvm.org/D103259