[flang] Fix crash in shape analysis of PACK()

[klausler]

Summary

A CHECK() was firing when a call to the PACK intrinsic does not have a
VECTOR= argument and at least one dimension of the shape of the ARRAY=
argument could not be determined. The CHECK was inappropriate, since
this can of course happen, such as when that argument is the result
of the SPREAD() intrinsic with non-constant DIM= or NCOPIES= arguments.
Replace with an if() statement.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Evaluate/shape.cpp

if (intrinsic->name == "shape" || intrinsic->name == "lbound" ||
       if (call.arguments().size() >= 3 && call.arguments().at(2)) {
         // SHAPE(PACK(,,VECTOR=v)) -> SHAPE(v)
         return (*this)(call.arguments().at(2));
       } else if (call.arguments().size() >= 2 && context_) {
         if (auto maskShape{(*this)(call.arguments().at(1))}) {
           if (maskShape->size() == 0) {
             // Scalar MASK= -> [MERGE(SIZE(ARRAY=), 0, mask)]
             if (auto arrayShape{(*this)(call.arguments().at(0))}) {
-              auto arraySize{GetSize(std::move(*arrayShape))};
-              CHECK(arraySize);
+              if (auto arraySize{GetSize(std::move(*arrayShape))}) {
                 ActualArguments toMerge{
                     ActualArgument{AsGenericExpr(std::move(*arraySize))},
                     ActualArgument{AsGenericExpr(ExtentExpr{0})},
                     common::Clone(call.arguments().at(1))};
                 auto specific{context_->intrinsics().Probe(
                     CallCharacteristics{"merge"}, toMerge, *context_)};
                 CHECK(specific);
                 return Shape{ExtentExpr{FunctionRef<ExtentType>{
                     ProcedureDesignator{std::move(specific->specificIntrinsic)},
                     std::move(specific->arguments)}}};
               }
+            }
           } else {
             // Non-scalar MASK= -> [COUNT(mask)]
             ActualArguments toCount{ActualArgument{common::Clone(
                 DEREF(call.arguments().at(1).value().UnwrapExpr()))}};
             auto specific{context_->intrinsics().Probe(
                 CallCharacteristics{"count"}, toCount, *context_)};
             CHECK(specific);
             return Shape{ExtentExpr{FunctionRef<ExtentType>{

flang/test/Evaluate/errors01.f90

subroutine s3
     x = eoshift(array, 1, [0, 0, 0], 2)
   end subroutine
   subroutine s4
     integer, parameter :: array(2,3) = reshape([(j, j=1, 6)], shape(array))
     logical, parameter :: mask(*,*) = reshape([(.true., j=1,3),(.false., j=1,3)], shape(array))
     integer :: x(3)
     !CHECK: error: Invalid 'vector=' argument in PACK: the 'mask=' argument has 3 true elements, but the vector has only 2 elements
     x = pack(array, mask, [0,0])
+    x = pack(spread(array, x(1), x(2)), .true.) ! regression check, once crashed
   end subroutine
   subroutine s5
     logical, parameter :: mask(2,3) = reshape([.false., .true., .true., .false., .false., .true.], shape(mask))
     integer, parameter :: field(3,2) = reshape([(-j,j=1,6)], shape(field))
     integer :: x(2,3)
     !CHECK: error: Invalid 'vector=' argument in UNPACK: the 'mask=' argument has 3 true elements, but the vector has only 2 elements
     x = unpack([1,2], mask, 0)
   end subroutine