Fix bug it GPU halo exchange initialization for continuation runs

Fix bug it GPU halo exchange initialization for continuation runs

Fixes a bug introduced in fdf8c9064e2289bb01b1536a06f83755450caee8
where, due to the presence of !(bFirstStep && ir->bContinuation) in
the if statement predicate of the block in which the init call was
located. In the case of continuation runs the halo exchange would not
be initialized until the second search step, causing the code to
crash. This change moves the init call to a separate block with the
correct logic in the if statement.