ir_expression_flattening: Fix breakage from hierarchichal visitor.

ir_expression_flattening: Fix breakage from hierarchichal visitor.

Similar to other situations where the visitor pattern doesn't fit, in
this case we need the pointer to the base instruction in the
instruction stream for where to insert any new instructions we
generate (not the instruction in the tree we're looking at).  By
removing the code for setting the base_ir, flattened expressions would
end up, for example, before the function definition where they had appeared.