nir: allow nir_lower_phis_to_scalar() on more src types

nir: allow nir_lower_phis_to_scalar() on more src types

Rather than only lowering if all srcs are scalarizable we instead
check that at least one src is scalarizable.

We change undef type to return false otherwise it will cause
regressions when it is the only scalarizable src.

total instructions in shared programs: 13219105 -> 13024547 (-1.47%)
instructions in affected programs: 1153797 -> 959239 (-16.86%)
helped: 581
HURT: 74

total cycles in shared programs: 333968972 -> 324807922 (-2.74%)
cycles in affected programs: 129809402 -> 120648352 (-7.06%)
helped: 571
HURT: 131

total spills in shared programs: 57947 -> 29130 (-49.73%)
spills in affected programs: 53364 -> 24547 (-54.00%)
helped: 351
HURT: 0

total fills in shared programs: 51310 -> 25468 (-50.36%)
fills in affected programs: 44882 -> 19040 (-57.58%)
helped: 351
HURT: 0

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>