Implement shufflevector builtin. (#36650)

This isn't _terribly_ useful as-is, because the only constant mask you can get at from Swift at present is the zeroinitializer, but even that is quite useful for optimizing the repeating: intializer on SIMD. At some future point we should wire up generating constant masks for the .even, .odd, .high and .low properties (and also eventually make shufflevector take non-constant masks in LLVM). But this is enough to be useful, so let's get it in.

lib/SILOptimizer/Transforms/AccessEnforcementReleaseSinking.cpp

@@ -120,6 +120,7 @@ static bool isBarrier(SILInstruction *inst) {
     case BuiltinValueKind::OnFastPath:
     case BuiltinValueKind::ExtractElement:
     case BuiltinValueKind::InsertElement:
+    case BuiltinValueKind::ShuffleVector:
     case BuiltinValueKind::StaticReport:
     case BuiltinValueKind::AssertConf:
     case BuiltinValueKind::StringObjectOr: