AArch64: Omit callframe setup/destroy when not necessary

Do not create CALLSEQ_START/CALLSEQ_END when there is no callframe to
setup and the callframe size is 0.

- Fixes an invalid callframe nesting for byval arguments, which would
  look like this before this patch (as in `big-byval.ll`):
    ...
    ADJCALLSTACKDOWN 32768, 0, ...   # Setup for extfunc
    ...
    ADJCALLSTACKDOWN 0, 0, ...  # setup for memcpy
    ...
    BL &memcpy ...
    ADJCALLSTACKUP 0, 0, ...    # destroy for memcpy
    ...
    BL &extfunc
    ADJCALLSTACKUP 32768, 0, ...   # destroy for extfunc

- Saves us two instructions in the common case of zero-sized stackframes.
- Remove an unnecessary scheduling barrier (hence the small unittest
  changes).

Differential Revision: https://reviews.llvm.org/D42006

llvm-svn: 322917