Add a flag to enable fine-grained optimize control.

Add a flag to enable fine-grained optimize control.

Defining this flag, `OPT_CONFIG`, enables a number of optimizer-related
configuration values. Changing these config values allows for some
interesting experiments without rebuilding the JIT (e.g. disabling the
SSA-based optimizer without forcing minopts, repeating optimizations,
etc.).

This flag is defined by default in debug builds and undefined by default
in release builds.

Commit migrated from https://github.com/dotnet/coreclr/commit/dc6aa70b0b0415229395a5456aed2f9623d3ba1b