[LV] Allow reductions with partial alias masking (#200813)
This patch enables reductions with partial alias masking by forcing the
use of a predicated reduction select with alias masking. This prevents
poison values from being propagated to the header phi for lanes outside
the alias mask.
Note: An alternate approach would be to select the identity value before
the final horizontal reduction (outside the vector loop). That however,
does not generalize to all reduction kinds (e.g., AnyOf), and at least
for AArch64 SVE, we already prefer the in-loop selects.
