Discovered while trying to build Julia with LLVM v13 (https://github.com/JuliaLang/julia/pull/42602).
While this shift is not defined (returns poison), it should not crash codegen,
as it may appear inside dead code (for example, a select instruction), and it
is legal as long as the value is not used.
Previously, this caused GlobalISel to emit invalid IR (a gpr32 to gpr64 copy) and fail during verification.
Tagging Amara as reviewer, as you wrote this code originally in rGcac1151845e6b .