[llvm] [ARM] CMSE security mitigation on function arguments and returned values (PR #89944)

[Oliver Hunt via llvm-commits]

ojhunt wrote:

Codegen logic seems reasonable, but I lack familiarity with CMSE - the only thing that gives me pause is the comment explaining the behaviour talks about extending <i32 but conceptually this applies to any integer return with a lower bit width than the resultant register, and the codegen does appear to correctly handle that

https://github.com/llvm/llvm-project/pull/89944