[TF:XLA] Add xla_dump_per_pass_hlo_proto_to flag and rename...

[TF:XLA] Add xla_dump_per_pass_hlo_proto_to flag and rename xla_dump_hlo_proto_to and xla_dump_prepass_hlo_proto_to.

Renamed:
xla_dump_hlo_proto_to -> xla_dump_optimized_hlo_proto_to
xla_dump_prepass_hlo_proto_to -> xla_dump_unoptimized_hlo_proto_to

xla_dump_per_pass_hlo_proto_to takes a directory from which to dump the serialized HLO
module protos after each HLO pass. This will help us compare HLO modules as
they change during HLO passes, such as what they evaluate to, or how fast they
perform.

The directory passed will contain multiple protos with the filename format:
module_<module id>.<pass number>.<pipeline_name>.after_<pass name>.pb
example:
module_0000.0009.simplification.after_dce.pb
such that pass number 0000_* is the first pass run, followed immediately by
pass number 0001_*.
PiperOrigin-RevId: 183700119