[flang] Don't emit false errors on LBOUND/UBOUND of assumed-rank arrays

Folding of LBOUND/UBOUND on assumed-rank arrays could lead to bogus errors
later.  This is a regression from a recent change intended to ensure that
patently bad DIM= arguments were caught even in the case of assumed-rank.
Rearrange the order of checks so that those bad DIM= argument values are
caught but otherwise defer LBOUND/UBOUND of assumed-rank arguments to
run time.  (It was tempting to always fold LBOUND(ar,DIM=n) to 1, but
that would obscure the run time error required when 'n' is out of bounds,
including the case of a scalar 'ar'.)

Differential Revision: https://reviews.llvm.org/D156755