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

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.

Differential Revision: https://reviews.llvm.org/D152212