i965: Convert live interval computation to using live variable analysis.

Our previous live interval analysis just said that anything in a loop
was live for the whole loop.  If you had to spill a reg in a loop,
then we would consider the unspilled value live across the loop too,
so you never made progress by spilling.  Eventually it would consider
everything in the loop unspillable and fail out.

With the new analysis, things completely deffed and used inside the
loop won't be marked live across the loop, so even if you
spill/unspill something that used to be live across the loop, you
reduce register pressure.  But you usually don't even have to spill
any more, since our intervals are smaller than before.

This fixes assertion failure trying to compile the shader for the
"glyphy" text rasterier and piglit glsl-fs-unroll-explosion.

Improves Unigine Tropics performance 1.3% +/- 0.2% (n=5), by allowing
more shaders to be compiled in 16-wide mode.

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