[llvm] [DAGCombiner] In mergeTruncStore, make sure we aren't storing shifted in bits. (PR #90939)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Note that we actually could merge the stores here, if we cared... just need to make sure we mask the value before we store it.

https://github.com/llvm/llvm-project/pull/90939