[AArch64] [Windows] Don't skip constructing UnwindHelp.

In certain cases, the first non-frame-setup instruction in a function is
a branch.  For example, it could be a cbz on an argument.  Make sure we
correctly allocate the UnwindHelp, and find an appropriate register to
use to initialize it.

Fixes https://bugs.llvm.org/show_bug.cgi?id=40184

Differential Revision: https://reviews.llvm.org/D58752

llvm-svn: 355136