[flang][hlfir] Do not emit extra declare for dummy used in BLOCK (#69184)

When a variable is used in a specification expression in a scope, it is
added to the list of variables that must be instantiated when lowering
the scope. When lowering a BLOCK, this caused instantiateVar to be
called again on all the host block variables appearing in block variable
specification expressions. This caused an extra declare to be emitted
for dummy inside block (for non dummy, instantiateVar is a no-op if the
symbol is already mapped).

Only call instantiateVar if the symbol is not mapped when lowering BLOCK
variables.