fix killing OP_PHI instructions

fix killing OP_PHI instructions

Currently kill_instruction() doesn't do anything with the
sources of OP_PHI instructions. But when these instructions
are removed the 'usage' of the associated sources must also
be removed. This is not done and as result the instructions
producing the phi-sources are not optimized away as expected.

This patch fixes that by calling clear_phi() when killing a
phi-instruction.

For example, when looking at the output of test-linearize,
the following function:
	void foo(int a, int *b, unsigned int g);
	void foo(int a, int *b, unsigned int g)
	{
		int d = 0;

		if ((!a || *b) && g)
			d = 16;
		else
			d = 8;
	}

gives this output without the patch:
	foo:
		br          %arg1, .L1, .L2
	.L1:
		phisrc.32   %phi1 <- $1
		br          .L3
	.L2:
		load.32     %r3 <- 0[%arg2]
		phisrc.32   %phi2 <- %r3
		br          .L3
	.L3:
		ret

The 'phisrc' instructions are obviously unneeded but nevertheless present.

With the patch, the output is much closer to what's expected:
	foo:
		br          %arg1, .L3, .L2
	.L2:
		load.32     %r3 <- 0[%arg2]
		br          .L3
	.L3:
		ret

Note 1) The 'load' instruction is also dead and should have been removed
but it's separate problem.

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