[BranchFolding] Allow hoisting to block with a single conditional branch.

Summary:
The BranchFolding pass is currently missing opportunities to hoist
common code if the hoisted-to block contains a single conditional branch
that has register uses.  This occurs somewhat frequently on AArch64 with
CBZ/TBZ opcodes.

This change also eliminates some code differences when debug info is
present since the presence of e.g. DBG_VALUE instructions in the
hoisted-to block can enable hoisting that wouldn't have occurred without
them.

Reviewers: MatzeB, rnk, kparzysz, twoh, aprantl, javed.absar

Subscribers: kristof.beyls, JDevlieghere, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D46324

llvm-svn: 332265