[llvm] [WebAssembly] Fix missed optimization in 50142 (PR #144741)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Can you [precommit](https://llvm.org/docs/InstCombineContributorGuide.html#precommit-tests) the test in this PR? I.e. rebase this so there's one commit adding the test without the changes, and another adding the changes to WebAssemblyISelLowering.cpp and the changes to the test case. That way you can see the diff in the PR

https://github.com/llvm/llvm-project/pull/144741