Allow scalar pred in select and scalar min/max in clamp.
Allow scalar pred in select and scalar min/max in clamp.

Semantics for both clamp and select allow restricted form of broadcasting, but it wasn't reflected in code.

For clamp, scalar min and/or max is allowed.
For select, scalar pred is allowed.

Context:
* https://openxla.org/xla/operation_semantics#clamp
* https://openxla.org/xla/operation_semantics#select
