Merge ix86_maybe_switch_abi with ix86_set_current_function

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.

	PR target/67850
	* config/i386/i386.c (ix86_maybe_switch_abi): Merged with ...
	(ix86_set_current_function): This.
	(TARGET_EXPAND_TO_RTL_HOOK): Removed.

From-SVN: r228565