Handle SLP build operand swapping for ternaries and calls

Handle SLP build operand swapping for ternaries and calls

The following adds SLP build operand swapping for .FMA which is
a ternary operator and a call.  The current code only handles
binary operators in assignments, thus the patch extends this to
handle both calls and assignments as well as binary and ternary
operators.

	* tree-vect-slp.cc (vect_build_slp_2): Handle ternary
	and call operators when swapping operands.

	* gcc.target/i386/vect-pr82426.c: Pass explicit -ffp-contract=fast.
	* gcc.target/i386/vect-pr82426-2.c: New testcase variant with
	-ffp-contract=on.