fix killing OP_CAST & friends

fix killing OP_CAST & friends

Currently  kill_instruction() doesn't do anything with the
operands of casts instructions. But when theses instructions
are removed the operands 'usage' must be adjusted and this is
not done and as result the instructions producing the operands
of these casts are not optimized away as expected.

This patch fixes that by killing these casts the same way as others
unary instructions (OP_NOT & OP_NEG).

To illustrate the situation, the output of test-linearize
on the following code:
	extern void __abort(void);
	struct s {
		int elem:3;
	};
	void foo(struct s *x);
	void foo(struct s *x)
	{
		if (x->elem == 0) {
			if (x->elem != 0 && x->elem != 1)
				__abort();
		}
	}

gives this output without the patch:
	foo:
		load.32     %r2 <- 0[%arg1]
		cast.32     %r3 <- (3) %r2
		br          .L1
	.L1:
		ret

Since x->elem can't be at the same time == 0 & != 0, the inner if is never
true and the whole code should have been optimized away.
The 'cast' instruction is obviously not needed but nevertheless present.

With the patch, the output is much closer to what's expected:
	foo:
		load.32     %r2 <- 0[%arg1]
		br          .L1
	.L1:
		ret

Note 1) The 'load' instruction is also dead but it's a separate problem.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>