genmatch: Avoid unused parameter warnings in generated code.

genmatch: Avoid unused parameter warnings in generated code.

This patch silences a number of unused parameter warnings whilst
compiling both generic-match.c and gimple-match.c.  The problem is
that multiple (polymorphic) functions are generated for generic_simplify
and gimple_simplify, each handling tree codes with a specific number
of children.  Currently, there are no simplifications for tree codes
with four or five children, leading to functions with "empty" bodies
and unused function arguments.  This patch detects those cases, and
generates stub functions (with anonymous arguments) to silence these
warnings.

2020-08-03  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* genmatch.c (decision_tree::gen): Emit stub functions for
	tree code operand counts that have no simplifications.
	(main): Correct comment typo.