Reader: defer lowering of certain constructs

[AndyAyersMS]

There will ultimately be many candidates but for now we can look at some of the float/int conversion operations. LLVM has opcodes for these that will lower to calls to the CRT; we want them to lower to calls to EE helpers instead.

See TODO.

[russellhadley]

@pgavlin Didn't we do this already as part of the CLR env work?

[pgavlin]

No: this requires that we specify a TargetLibraryInfo instance to the backend. There is not currently any way to do this outside of changing our ORC layers s.t. we can add passes the pass manager.