[mlir][LLVMIR target] Fix llvm.freeze builder to prevent crashes

[krzysz00]

Summary

The freeze builder did not assign the result of creating the freeze
operation to $res, which meant that when subsequent translations (such
as a sext) tried to use that result or query its type, mlir-translate
would crash.

This fixes the issue and adds a test for it.

mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td

def LLVM_SelectOp
   }];
 }
 def LLVM_FreezeOp : LLVM_Op<"freeze", [SameOperandsAndResultType]> {
   let arguments = (ins LLVM_Type:$val);
   let results = (outs LLVM_Type:$res);
   let builders = [LLVM_OneResultOpBuilder];
   let assemblyFormat = "$val attr-dict `:` type($val)";
   string llvmInstName = "Freeze";
-  string llvmBuilder = "builder.CreateFreeze($val);";
+  string llvmBuilder = "$res = builder.CreateFreeze($val);";
   string mlirBuilder = [{
     $res = $_builder.create<LLVM::FreezeOp>($_location, $val);
   }];
 }
 
 // Terminators.
 def LLVM_BrOp : LLVM_TerminatorOp<"br",
     [DeclareOpInterfaceMethods<BranchOpInterface>, Pure]> {

mlir/test/Target/LLVMIR/llvmir.mlir

llvm.func @callFreezeOp(%x : i32) {
   // CHECK: freeze i32 %{{[0-9]+}}
   %0 = llvm.freeze %x : i32
   %1 = llvm.mlir.undef : i32
   // CHECK: freeze i32 undef
   %2 = llvm.freeze %1 : i32
   llvm.return
 }
 
+// CHECK-LABEL: @freezeUsed
+llvm.func @freezeUsed(%x : i32) -> i64 {
+  // CHECK: %[[frozen:.*]] = freeze i32
+  %frozen = llvm.freeze %x : i32
+  // CHECK: %[[ext:.*]] = sext i32 %[[frozen]] to i64
+  %ext = llvm.sext %frozen : i32 to i64
+  // CHECK: ret i64 %[[ext]]
+  llvm.return %ext : i64
+}
+
 // CHECK-LABEL: @boolConstArg
 llvm.func @boolConstArg() -> i1 {
   // CHECK: ret i1 false
   %0 = llvm.mlir.constant(true) : i1
   %1 = llvm.mlir.constant(false) : i1
   %2 = llvm.and %0, %1 : i1
   llvm.return %2 : i1
 }