[flang][hlfir] Simplify hlfir::convertToValue

Use hlfir::loadTrivialScalars to dereference pointer, allocatables, and
load numerical and logical scalars.

This has a small fallout on tests:

- load is done on the HLFIR entity (#0 of hlfir.declare) and not the FIR one (#1). This makes no difference at the FIR level (#1 and #0 only differs to account for assumed and explicit shape lower bounds).

- loadTrivialScalars get rids of allocatable fir.box for monomoprhic scalars
  (it is not needed). This exposed a bug in lowering of MERGE with
  a polymorphic and a monomorphic argument: when the monomorphic is not
  a fir.box, the polymorphic fir.class should not be reboxed but its
  address should be read.

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D153252