[clang] [clang][CodeGen] Fix in codegen for __builtin_popcountg/ctzg/clzg (PR #90845)

[Björn Pettersson via cfe-commits]

bjope wrote:

> Please add a testcase for i1/i2 (which are the only cases where this actually matters).

I added tests in a new file (builtins-bitint.c) to show that we get expected results for a target that isn't treating a zero input to ctzg/clzg as poison.

https://github.com/llvm/llvm-project/pull/90845