AArch64 GIsel: legalize lshr operands, even if it is poison

Previously, this caused GlobalISel to emit invalid IR (a gpr32 to gpr64
copy) and fail during verification.

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 IR input, as long as the value is unused.

Discovered while trying to build Julia with LLVM v13:
https://github.com/JuliaLang/julia/pull/42602.

Reviewed By: aemerson

Differential Revision: https://reviews.llvm.org/D114389