[flang][LoopVersioning] support fir.declare

When FIR comes from HLFIR, there will be a fir.declare operation between
the source and the usage of each source variable (and some temporary
allocations). This pass needs to be able to follow these so that it can
still transform loops when HLFIR is used, otherwise it mistakenly
assumes these values are not function arguments.

More work is needed after this patch to fully support HLFIR, because the
generated code tends to use fir.array_coor instead of fir.coordinate_of.

Differential Revision: https://reviews.llvm.org/D157964