[clang] [llvm] [APInt] Consistently use the terms pdep/pext instead of expandBits/compressBits (PR #205112)

[Jan Schultke via cfe-commits]

eisenwave wrote:

Using `compressBits` was deliberate from the start in order to align with `std::bit_compress` (now in C++29) and matematical lingo.

But I guess that `APInt` is the wrong place for that, given that operations on it are supposed to mirror the `@llvm.*` operations on integers, so it seems better to call it `pext` and `pdep` here.

https://github.com/llvm/llvm-project/pull/205112