Merge ix86_maybe_switch_abi with ix86_set_current_function
ix86_maybe_switch_abi is called to late during RTL expansion and we
use the stale information from compilation of the previous function.
aggregate_value_p uses call_used_regs. aggregate_value_p is used by
IPA and return value optimization, which are called before
ix86_maybe_switch_abi is called. This patch merges ix86_maybe_switch_abi
with ix86_set_current_function.