Ensure emit_move_insn operands are valid

Some architectures are fine with PLUS in move instructions, but others
are not (amdgcn is the motivating example).

2021-05-07  Jakub Jelinek  <jakub@redhat.com>
	    Andrew Stubbs  <amd@codesourcery.com>

gcc/ChangeLog:

	PR target/100418
	* builtins.c (try_store_by_multiple_pieces): Use force_operand for
	emit_move_insn operands.