[llvm] [AArch64] Avoid overflow when using shl lower mul (PR #97148)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Thanks for looking into this. Could we change it to a check for shift amount > bitwidth? It would feel a little more reliable than checking if the result was folded to undef.

https://github.com/llvm/llvm-project/pull/97148