ignore VOID when trying to if-convert phi-nodes

ignore VOID when trying to if-convert phi-nodes

Simple if-then-else expressions can often be
replaced by an OP_SELECT. This is done in a very
generic way by looking not at the test/if part but
at OP_PHIs which contain exactly 2 values.

An implementation detail makes that the address
of valid OP_PHI's sources must not change, so the
list holding these values must not be repacked and such.
As consequence, when a value need to be removed from the
list this value is simply replaced in the list by a VOID.
These VOIDs must then be ignored when processing OP_PHI's
sources.
But for the if-conversion, these VOID are not ignored
and are thus considered like any other value which in turn
make miss some optimization opportunities.
For example code like:
	int foo(int a)
	{
		if (a)
			return 0;
		else
			return 1;
		return 2;
	}

will be linearized into something like:
	...
	phi.32      %r2 <- %phi1, %phi2, VOID
	ret.32      %r2
where %phi1 & %phi2 correspond to the 'return 0' & 'return 1'
and the VOID correspond to the dead 'return 2'.
This code should be trivially be converted to a select
but is not because the OP_PHI is considered as having 3
elements instead of 2.

Worse, if at some moment we would have a phi list with:
	phi.32      %r2 <- %phi1, VOID
then the optimization would trigger but the corresponding
code make the assumption that the pseudos are the target
of an OP_PHISOURCE instruction, which VOIDs, of course,
are not and a crash should ensue.

Fix this by filtering out these VOIDs before checking the
conditions of the if-conversion.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>