Move DroppedVariableStats code to its own file and change the class to
have an extensible design so that we can use it to add dropped
statistics to MIR passes and the instruction selector.

Also moved class DroppedVariableStatsIR to its own file.

Reland 2de7881

(cherry picked from commit 5717a99)

(cherry picked from commit 0b5b09b)

This patch uses the DroppedVariableStats class to add dropped variable
statistics for MIR passes.

Reland 1c082c9

(cherry picked from commit 3bf91ad)

The DroppedVariableStats::calculateDroppedStatsAndPrint should check if
it's InlinedAts stack contains the the function name that is being
accessed to make sure that a pass did not create a new function
declaration which may then lead to a crash. For example, in hot-cold
splitting, the Module before the pass will not contain the newly created
cold function and can cause a crash when trying to access the InlinedAts
stack with the function name of the newly created cold function.

(cherry picked from commit e433fc3)