[SVE] Eliminate calls to default-false VectorType::get() from mlir

[ctetreau]

rebase

mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp

LLVMType LLVMType::setStructTyBody(LLVMType structType,
   return getLocked(&structType.getDialect(), [=] {
     st->setBody(llvmElements, isPacked);
     return st;
   });
 }
 LLVMType LLVMType::getVectorTy(LLVMType elementType, unsigned numElements) {
   // Lock access to the dialect as this may modify the LLVM context.
   return getLocked(&elementType.getDialect(), [=] {
-    return llvm::VectorType::get(elementType.getUnderlyingType(), numElements);
+    return llvm::FixedVectorType::get(elementType.getUnderlyingType(),
+                                      numElements);
   });
 }
 
 LLVMType LLVMType::getVoidTy(LLVMDialect *dialect) {
   return dialect->impl->voidTy;
 }
 
 bool LLVMType::isVoidTy() { return getUnderlyingType()->isVoidTy(); }