Improvements in pmgen

[cliffordwolf]

This adds some improvements to pmgen for recursive patterns. It also includes a demo_reduce pass to demonstrate the new features. (Not sure if that should be included in the final PR.)

Simple test case for demo_reduce (test.v):

module test (input A, B, C, D, E, output X, Y, Z);
	assign X = (A & B) & (C & D & E);
	assign Y = (A | B | C | D) | E;
	assign Z = A ^ (B ^ C) ^ D ^ E;
endmodule

Run with yosys -p 'prep; simplemap;; demo_reduce; show' test.v.

[eddiehung]

• Document semioptional (is it only valid inside a subpattern?)
• Add CHANGELOG entry

[cliffordwolf]

Demo for the test case generator:

yosys -p 'test_pmgen -generate reduce; hierarchy -top pmtest_test_pmgen_pm_reduce_00099; test_pmgen -reduce_chain;; show'

[eddiehung]

Can we have a test that runs test_pmgen on a circuit?