For pre-ra scheduling, we use two options `-misched-topdown` and
`-misched-bottomup` to force the direction.

While for post-ra scheduling, we use `-misched-postra-direction`
with enumerated values (`topdown`, `bottomup` and `bidirectional`).

This is not unified and adds some mental burdens. Here we replace
these two options `-misched-topdown` and `-misched-bottomup` with
`-misched-prera-direction` with the same enumerated values.

To avoid the condition of `getNumOccurrences() > 0`, we add a new
enum value `Unspecified` and make it the default initial value.

These options are hidden, so we needn't keep the compatibility.

Rename postprocessDAG to camel case.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D146795

Some cl::ZeroOrMore were added to avoid the `may only occur zero or one times!`
error. More were added due to cargo cult. Since the error has been removed,
cl::ZeroOrMore is unneeded.

Also remove cl::init(false) while touching the lines.

This is a (fixed) recommit of https://reviews.llvm.org/D121169

after:  1061034926
before: 1063332844

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D121681