a new i-cache metric

Summary:
The diff introduces two measures for i-cache performance: a TSP measure (currently used for optimization) and an "extended" TSP measure that takes into account jumps between non-consecutive basic blocks. The two measures are computed for estimated addresses/sizes of basic blocks and for the actually omitted addresses/sizes.

Intuitively, the Extended-TSP metric quantifies the expected number of i-cache misses for a given ordering of basic blocks. It has 5 parameters:
 - FallthroughWeight is the impact of fallthrough jumps on the score
 - ForwardWeight is the impact of forward (but not fallthrough) jumps
 - BackwardWeight is the impact of backward jumps
 - ForwardDistance is the max distance of a forward jump affecting the score
 - BackwardDistance is the max distance of a backward jump affecting the score
We're still learning the "best" values for the options but default values look reasonable so far.

(cherry picked from FBD6331418)