[clang] [CIR] Support building vectors from smaller vectors (PR #231404)

[Arseniy Obolenskiy via cfe-commits]

================
@@ -2948,6 +2952,45 @@ mlir::Value ScalarExprEmitter::VisitMemberExpr(MemberExpr *e) {
   return emitLoadOfLValue(e);
 }
 
+mlir::Value
+ScalarExprEmitter::emitVectorInitWithSubvectors(mlir::Location loc,
+                                                cir::VectorType vectorType,
+                                                ArrayRef<mlir::Value> inits) {
+  const unsigned resElts = vectorType.getSize();
+  mlir::Value result =
+      builder.getConstant(loc, cir::PoisonAttr::get(vectorType));
+  unsigned curIdx = 0;
+  for (mlir::Value init : inits) {
----------------
aobolensk wrote:

"this should never happen" (c)

Let's add an assertion for that 

https://github.com/llvm/llvm-project/pull/231404