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

Authored by efriedma on Feb 28 2019, 12:38 PM.

Description

[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