[llvm] [AArch64][GlobalISel] Fix incorrect ABI when tail call not supported (PR #70215)

[via llvm-commits]

tschuett wrote:

The documentation of `determineAssignments` states that it manipulates its arguments. This looks hacky. Probably `determineAssignments` should return an optional instead. Mutable arguments are an anti pattern.

https://github.com/llvm/llvm-project/pull/70215