Make LiveRangeQuery work for PHIDefs as well.

If a PHI value happens to be live out from the layout predecessor of its
def block, the def slot index will be in the middle of the segment:

%vreg11 = [192r,240B:0)[352r,416B:2)[416B,496r:1)  0@192r 1@480B-phi %2@352r

A LiveRangeQuery for 480 should return NULL from valueIn() since the
PHI value is defined at the block entry, not live in to the block.

No test case, future code depends on this functionality.

llvm-svn: 163971