[fir] Remove `fir.unbox` operation

[clementval]

Summary

fir.unbox operation is an old operation that is no longer required.
There are couple of other operations that can be used to extract
information from a fir.box such as fir.box_rank, fir.box_addr,
fir.box_dims.

This was found during the upstreaming process.

[awarzynski]

LGTM, always nice to see some unused code being removed!

flang/include/flang/Optimizer/Dialect/FIROps.td

def fir_EmboxProcOp : fir_Op<"emboxproc", [NoSideEffect]> {
 
   let parser = "return parseEmboxProcOp(parser, result);";
 
   let printer = "::print(p, *this);";
 
   let verifier = "return ::verify(*this);";
 }
 
-def fir_UnboxOp : fir_SimpleOp<"unbox", [NoSideEffect]> {
-  let summary = "unbox the boxed value into a tuple value";
-
-  let description = [{
-    Unbox a boxed value into a result of multiple values from the box's
-    component data.  The values are, minimally, a reference to the data of the
-    entity, the byte-size of one element, the rank, the type descriptor, a set
-    of flags (packed in an integer, and an array of dimension information (of
-    size rank).
-
-    ```mlir
-      %40   = ... : !fir.box<!fir.type<T>>
-      %41:6 = fir.unbox %40 : (!fir.box<!fir.type<T>>) -> (!fir.ref<!fir.type<T>>, i32, i32, !fir.tdesc<!fir.type<T>>, i32, !fir.array<? x index>)
-    ```
-  }];
-
-  let arguments = (ins fir_BoxType:$box);
-
-  let results = (outs
-    fir_ReferenceType,  // pointer to data
-    AnyIntegerLike,     // size of a data element
-    AnyIntegerLike,     // rank of data
-    fir_TypeDescType,   // abstract type descriptor
-    AnyIntegerLike,     // attribute flags (bitfields)
-    fir_SequenceType    // dimension information (if any)
-  );
-}
-
 def fir_UnboxCharOp : fir_SimpleOp<"unboxchar", [NoSideEffect]> {
   let summary = "unbox a boxchar value into a pair value";
 
   let description = [{
     Unboxes a value of `boxchar` type into a pair consisting of a memory
     reference to the CHARACTER data and the LEN type parameter.
 
     ```mlir

flang/test/Fir/fir-ops.fir

 
 // CHECK-LABEL: func @boxing_match() {
 func @boxing_match() {
 // CHECK: [[VAL_38:%.*]] = fir.alloca i32
 // CHECK: [[VAL_39:%.*]] = fir.alloca !fir.type<qq2{f1:i32,f2:f64}>
 // CHECK: [[VAL_40:%.*]] = fir.alloca !fir.char<1>
 // CHECK: [[VAL_41:%.*]] = fir.alloca tuple<i32, f64>
 // CHECK: [[VAL_42:%.*]] = fir.embox [[VAL_38]] : (!fir.ref<i32>) -> !fir.box<i32>
-// CHECK: [[VAL_43:%.*]]:6 = fir.unbox [[VAL_42]] : (!fir.box<i32>) -> (!fir.ref<i32>, i32, i32, !fir.tdesc<i32>, i32, !fir.array<3x?xindex>)
 // CHECK: [[VAL_44:%.*]] = arith.constant 8 : i32
 // CHECK: [[VAL_45:%.*]] = fir.undefined !fir.char<1>
 // CHECK: [[VAL_46:%.*]] = fir.emboxchar [[VAL_40]], [[VAL_44]] : (!fir.ref<!fir.char<1>>, i32) -> !fir.boxchar<1>
 // CHECK: [[VAL_47:%.*]]:2 = fir.unboxchar [[VAL_46]] : (!fir.boxchar<1>) -> (!fir.ref<!fir.char<1>>, i32)
 // CHECK: [[VAL_48:%.*]] = fir.undefined !fir.type<qq2{f1:i32,f2:f64}>
 // CHECK: [[VAL_49:%.*]] = arith.constant 0 : i32
 // CHECK: [[VAL_50:%.*]] = arith.constant 12 : i32
 // CHECK: [[VAL_51:%.*]] = fir.insert_value [[VAL_48]], [[VAL_50]], [0 : i32] : (!fir.type<qq2{f1:i32,f2:f64}>, i32) -> !fir.type<qq2{f1:i32,f2:f64}>

 // CHECK: fir.call @store_tuple([[VAL_61]]) : (tuple<!fir.type<qq1{f1:i32}>>) -> ()
 // CHECK: return
 // CHECK: }
   %0 = fir.alloca i32
   %d6 = fir.alloca !fir.type<qq2{f1:i32,f2:f64}>
   %d3 = fir.alloca !fir.char<1>
   %e6 = fir.alloca tuple<i32,f64>
   %1 = fir.embox %0 : (!fir.ref<i32>) -> !fir.box<i32>
-  %2:6 = fir.unbox %1 : (!fir.box<i32>) -> (!fir.ref<i32>,i32,i32,!fir.tdesc<i32>,i32,!fir.array<3x?xindex>)
   %c8 = arith.constant 8 : i32
   %3 = fir.undefined !fir.char<1>
   %4 = fir.emboxchar %d3, %c8 : (!fir.ref<!fir.char<1>>, i32) -> !fir.boxchar<1>
   %5:2 = fir.unboxchar %4 : (!fir.boxchar<1>) -> (!fir.ref<!fir.char<1>>, i32)
   %6 = fir.undefined !fir.type<qq2{f1:i32,f2:f64}>
   %z = arith.constant 0 : i32
   %c12 = arith.constant 12 : i32
   %a2 = fir.insert_value %6, %c12, [0 : i32] : (!fir.type<qq2{f1:i32,f2:f64}>, i32) -> !fir.type<qq2{f1:i32,f2:f64}>