[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
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index bb06517..b060ec7 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -3231,6 +3231,11 @@
   case PPC::BI__builtin_tabortdci:
     return SemaBuiltinConstantArgRange(TheCall, 0, 0, 31) ||
            SemaBuiltinConstantArgRange(TheCall, 2, 0, 31);
+  case PPC::BI__builtin_altivec_dst:
+  case PPC::BI__builtin_altivec_dstt:
+  case PPC::BI__builtin_altivec_dstst:
+  case PPC::BI__builtin_altivec_dststt:
+    return SemaBuiltinConstantArgRange(TheCall, 2, 0, 3);
   case PPC::BI__builtin_vsx_xxpermdi:
   case PPC::BI__builtin_vsx_xxsldwi:
     return SemaBuiltinVSX(TheCall);