Add liveness analysis to optimise local var init

[drcjt]

Code generator emits code to initialise locals as part of method prolog, see CodeGenerator.GenerateProlog(..). Similarly the LocalHeapCodeGenerator.GenerateCode(..) does initialisation of localloc memory.

Currently the entire of the locals/localloc is initialised - this is suboptimal if assignments are made to the locals before use. To detect this liveness analysis needs to be implemented and used to set the LocalVariableDescriptor.MustInit flag. The init code can then just init the vars that have been marked as needing it.

[drcjt]

Simple example to use for testing, note this is using unsafe code:

int i;      // must be init
int j = *&i;    // no need to init j
Console.WriteLine(j);

[drcjt]

For structs generally IL will contain a ldloca before a stfld or a ldloc before a ldfld. For the store the ldloca will be interpreted in the liveness analysis as a use of the struct thus resulting in the struct being marked as must-init. So currently the prolog code will be zeroing the struct.

If a constructor is used for the struct then potentially there is double initialization happening. However the current implementation is conservative and does not try to deal with this.

[drcjt]

In terms of testing zero initialization of locals - it looks like these are the tests in dotnet runtime: https://github.com/dotnet/runtime/tree/0916f7ba98716b18ccfa60081befa164bec3c203/src/tests/JIT/Directed/zeroinit