Reduce nesting of parentheses in conditionals generated by genattrtab

gcc/ChangeLog:

	* genattrtab.c (write_test_expr): New parameter EMIT_PARENS
	which defaults to true.  Emit an outer pair of parentheses only if
	EMIT_PARENS.  When continuing a chain of && or || (or & or |),
	don't emit parentheses for the right-hand operand.

From-SVN: r235536