[llvm] [IR][ASMParser] Use default AS for alloca without explicit AS (PR #135786)

[Shilei Tian via llvm-commits]

shiltian wrote:

> Sounds like we are looking into similar fuzzer tickets :)

Yeah, that was motivated by some fuzzer tickets which turns out to be invalid IR.

> Be aware that this probably needs to go into some kind of target-specific verifier, since there seem to be targets (for example WASM, according to https://reviews.llvm.org/D101045 ) that use non-default address spaces in allocas.

Since we don't have a target specific verifier at the moment, I'm gonna just do it in the common one, and put a FIXME there when I hard code AS5 instead of `AMDGPUAS::PRIVATE_ADDRESS`, since we can't include the header there.

https://github.com/llvm/llvm-project/pull/135786