ARM: When re-creating a branch via InsertBranch, preserve CPSR flags.

ARM: When re-creating a branch via InsertBranch, preserve CPSR flags.

In particular, this preserves the kill flag, which allows the Thumb2 cbn?z
optimization to be applied in cases where a branch has been re-created after
the live variables analysis pass, e.g. by the machine block placement pass.

This appears to be low risk; a number of other targets seem to already be
doing something similar, e.g. AArch64, PowerPC.

Differential Revision: http://reviews.llvm.org/D9184

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235639 91177308-0d34-0410-b5e6-96231b3b80d8