Compute the critical path length through a trace.

Compute the critical path length through a trace.

Whenever both instruction depths and instruction heights are known in a
block, it is possible to compute the length of the critical path as
max(depth+height) over the instructions in the block.

The stored live-in lists make it possible to accurately compute the
length of a critical path that bypasses the current (small) block.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161197 91177308-0d34-0410-b5e6-96231b3b80d8