[llvm] [LLVM] BasicTTIImpl allow unknown type during legality checking (PR #89848)

[Kohei Asano via llvm-commits]

khei4 wrote:

LGTM. But I think it's better to be reviewed by recently active committers. 
@nikic @dtcxzyw @DianQK 

https://github.com/llvm/llvm-project/pull/89848