[RuntimeUnroll] NFC: Add a profitability function for mutliexit loop

Separated out the profitability from the safety analysis for multiexit
loop unrolling. Currently, this is an NFC because profitability is true
only if the unroll-runtime-multi-exit is set to true (off-by-default).

This is to ease adding the profitability heuristic up for review at
D35380.

llvm-svn: 308753