[flang] Fix shape analysis of RESHAPE result

Shape analysis of RESHAPE(..., SHAPE=s) should of course return
the SHAPE= actual argument when it is constant; but when it is
not, its length is still known, and thus so is the rank of the
result of RESHAPE(), and shape analysis should at least return
a shape vector of the right length rather than a result that
makes the result appear to be a scalar, which can lead to some
bogus error messages.

Also, while here: rename a private GetShapeHelper::AsShape()
routine so that it can't be confused with the ones in the API
of shape.h.

Differential Revision: https://reviews.llvm.org/D123712