[LV] Handle partial reductions with predication (#194859)

Enables partial reductions to be used when a reduction is gated by a
condition inside a loop.

Previously when a loop contained a condition, this would introduce a
BLEND to the chain, which prevented a partial reduction from being used
in a pattern where it would normally be created.

This patch records the blend as part of the partial reduction chain and
recognizes blends that select between the updated reduction value and
the previous accumulator.

For tail-folded loops, the transform also combines the loop active-lane
mask with the blend condition, so the generated partial reduction is
predicated by both the tail-folding mask and the original loop
condition. This allows partial reductions to also be used for predicated
reductions with tail folding enabled.