Add IR size after certain phases to the JIT log.

Add IR size after certain phases to the JIT log.

This change adds support for measuring and reporting the number of nodes
in a function after a certain subset of the JIT's phases. This reporting
is enabled by setting `COMPlus_JitMeasureIR` and `COMPlus_JtiTimeLogCsv`
in conjunction with each other.

Commit migrated from https://github.com/dotnet/coreclr/commit/a274b5af4bdc6af0919a28708b3085a59da32cd1