[clang] Revert "[X86][clang] Lift _BitInt() supported max width." (PR #81175)

[John McCall via cfe-commits]

rjmccall wrote:

There's no such thing as "this is impossible to do".  Clang, as the frontend, is responsible for emitting IR that has gets the effect we want.  If that means contorting the IR we generate to do ugly things like memcpying into a temporary, that's our life.

I am not surprised that we do not get any sort of reliable behavior from backends for `iNNN` when that's not architectural-legal.  I'm pretty sure I raised that concern during the review.

https://github.com/llvm/llvm-project/pull/81175