[APInt.h] Reduce the APInt header file interface a bit. NFC

This moves one mid-size function out of line, inlines the
trivial tcAnd/tcOr/tcXor/tcComplement methods into their only
caller, and moves the magic/umagic functions into SelectionDAG
since they are implementation details of its algorithm. This
also removes the unit tests for magic, but these are already
tested in the divide lowering logic for various targets.

This also upgrades some C style comments to C++.

Differential Revision: https://reviews.llvm.org/D109476