[CodeGen] Reuse BlockUtils for -unreachableblockelim pass (NFC)

Summary:
The logic in the -unreachableblockelim pass does the following:

1. It traverses the function it's given in depth-first order and
   creates a set of basic blocks that are unreachable from the
   function's entry node.
2. It iterates over each of those unreachable blocks and (1) removes any
   successors' references to the dead block, and (2) replaces any uses of
   instructions from the dead block with null.

The logic in (2) above is identical to what the `llvm::DeleteDeadBlocks`
function from `BasicBlockUtils.h` does. The only difference is that
`llvm::DeleteDeadBlocks` replaces uses of instructions from dead blocks
not with null, but with undef.

Replace the duplicate logic in the -unreachableblockelim pass with a
call to `llvm::DeleteDeadBlocks`. This results in less code but no
functional change (NFC).

Reviewers: mkazantsev, wmi, davidxl, silvas, davide

Reviewed By: davide

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59064

llvm-svn: 355634