[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