Support multiple operand counts for .md @ patterns

Support multiple operand counts for .md @ patterns

This patch extends the support for "@..." pattern names so that
the patterns can have different numbers of operands.  This allows
things like binary and ternary operations to be handled in a
consistent way, a bit like optabs.  The generators assert that
the number of operands passed is correct for the underlying
instruction.

Also, replace_operands_with_dups iterated over the old rtx format
even after having decided to do a replacement, which broke with
match_operator.

2019-07-12  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* doc/md.texi: Document that @ patterns can have different
	numbers of operands.
	* genemit.c (handle_overloaded_gen): Handle this case.
	* genopinit.c (handle_overloaded_gen): Likewise.
	* gensupport.c (replace_operands_with_dups): Iterate over
	the new rtx's format rather than the old one's.

From-SVN: r273432