[PowerPC][Altivec][Clang] Check compile-time constant for vec_dst*

Summary:
This is follow up of https://reviews.llvm.org/D66699.
We might get ISEL ICE if we call vec_dss with non const 3rd arg.

```
Cannot select: intrinsic %llvm.ppc.altivec.dst
```

We should check the constraints in clang and generate better error
messages.

Reviewers: nemanjai, hfinkel, echristo, #powerpc, wuzish

Reviewed By: #powerpc, wuzish

Subscribers: wuzish, kbarton, MaskRay, shchenz, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66748

llvm-svn: 370912