[CodeGen] Don't zero callee-save registers with zero-call-used-regs (PR57692)

Callee save registers must be preserved, so -fzero-call-used-regs
should not be zeroing them. The previous implementation only did
not zero callee save registers that were saved&restored inside the
function, but we need preserve all of them.

Fixes https://github.com/llvm/llvm-project/issues/57692.

Differential Revision: https://reviews.llvm.org/D133946