do not spill dead phis

Regalloc will be able to handle these spurious phis,
however, some other spurious dead instructions can be
emitted.  It would be better to get rid of them upfront;
maybe by modifying isel, or by inserting a proper dce
pass.  An example of that undesirable behavior is
exposed below.

@l1
	%foo =w ...
@l2
	%dead =w phi @l1 %foo, ...

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