[Polly][SchedOpt] Account for prevectorization of multiple statements.

[Polly][SchedOpt] Account for prevectorization of multiple statements.

A prevectorized loop may contain multiple statements, in which case
isl_schedule_node_band_sink will sink the vector band to multiple
leaves. Instead of statically assuming a specific tree structure after
sinking, add a SIMD marker to all inner bands.

Fixes llvm.org/PR52637