[BPF] lowering target address leaf nodes tconstpool

[inclyc]

Adds custom lowering for these leaf nodes.

Please ref: #73668 for test coverage

[tamird]

Please add tests.

[inclyc]

Please add tests.

These nodes are basic symbols and I don't know how to test it directly, however #73668 referenced tconstpool, would you like to see other nodes get test-coveraged in this way?

(I checked commits in other targets and they seem to commit these pattern without tests, e.g. bec7b16)

[eddyz87]

The code lgtm.
Constants pool usage matches other backends (e.g. x86) and #73668 triggers code paths from this MR.
It seems that there is no straightforward way to test this commit: all uses of getConstantPool/getTargetConstantPool I checked are internal to DAG translation of specific constructs.
It might be possible to make a unit test that constructs DAG and lowers it, but I think that's an overkill.

@yonghong-song , wdyt?

[inclyc]

Kindly ping @yonghong-song 😃

This PR has been blocked for 1 month 😨 ! Could you please take a look?

[inclyc]

I'd like to merge this now :)

Please comment here if there are any issue caused by this PR, and I would happy to revert it!