[PR target/116720] Fix test for valid mempair operands

So this BZ is a case where we incorrectly indicated that the operand array was
suitable for the t-head load/store pair instructions.

In particular there's a test which checks alignment, but that happens *before*
we know if the operands are going to be reversed.  So the routine reported the
operands are suitable.

At a later point the operands have been reversed into the proper order and we
realize the alignment test should have failed, resulting in the unrecognized
insn.

This fixes the code by moving the reversal check earlier and actually swapping
the local variables with the operands.  That in turn allows for simpler testing
of alignments, ordering, etc.

I've tested this on rv32 and rv64 in my tester.  I don't offhand know if the
patch from Filip that's been causing headaches for the RISC-V port has been
reverted/fixed.  So there's a nonzero chance the pre-commit CI tester will
fail.  I'll keep an eye on it and act appropriately.

	PR target/116720
gcc/
	* config/riscv/thead.cc (th_mempair_operands_p): Test for
	aligned memory after swapping operands.  Simplify test for
	first memory access as well.

gcc/testsuite/
	* gcc.target/riscv/pr116720.c: New test.