Add new cond-rw pass to have forward propagation track cond branches

Add new cond-rw pass to have forward propagation track cond branches

Add a new pass to rewrite conditional branches.  This is introducing
and placing a new LIMPLE operator 'assume' in use by fwprop to
propagate conditional branch test informations on target basic blocks.

	* lisp/emacs-lisp/comp.el (comp-passes): Add `comp-cond-rw'.
	(comp-limple-assignments): Add `assume' operator.
	(comp-emit-assume, comp-cond-rw-target-slot, comp-cond-rw-func)
	(comp-cond-rw): Add new functions.
	(comp-fwprop-insn): Update to pattern match `assume' insns.
	* src/comp.c (emit_limple_insn): Add for `assume'.
	(syms_of_comp): Define 'Qassume' symbol.