[X86][Darwin] Emit compact-unwind for register-sized stack adjustments
For stack frames on the size of a register in x86, a code size optimization
emits "push rax/eax" instead of "sub" for stack allocation. For example:
.cfi_def_cfa_offset 16 ... .cfi_endproc
However, we are falling back to DWARF in this case because we cannot
encode %rax as a saved register.
This requirement is wrong, since we don't care about the contents of
%rax, it is the equivalent of a sub.
In order to specify that we care about the contents of %rax, we would
need a .cfi_offset %rax, <offset>.
It's also overzealous in the case where there are pushes for callee saved
registers followed by a "push rax/eax" instead of "sub", in which case we should
also be able to encode the callee saved regs and everything else using compact
Patch authored by Bruno Cardoso Lopes.
Differential Revision: https://reviews.llvm.org/D13793