[NFC] Include PassID for runBeforePass children.

Debugging the dropped variable statistics for large LLVM IR files can be
made easier if the functions called before an optimization pass is run
inclu

[NFC] Include PassID for runBeforePass children.

Debugging the dropped variable statistics for large LLVM IR files can be
made easier if the functions called before an optimization pass is run
includes the PassID of the pass that will be run after statistics
metrics are collected. This patch adds that support.

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[NFC] Move DroppedVariableStats to its own file (#120711)

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
stat

[NFC] Move DroppedVariableStats to its own file (#120711)

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 2de78815604e9027efd93cac27c517bf732587d2

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