JIT optimization: IV widening

[JosephTremoulet]

(I'm creating tracking issues for some optimizations that RyuJit doesn't perform, so we'll have a place to reference/note when we see the lack of them affecting particular benchmarks)

[JosephTremoulet]

This leaves extra converts in BenchI-XPos_Mtx and IniArray (and basically every loop that indexes into arrays...)

[mikedn]

Dup of #7312

[mikedn]

An alternative solution for array indices might be to change signed casts to unsigned casts, I once tried that in dotnet/coreclr#8821 and it does improve performance in some cases. That approach unfortunately produced some regressions but there may be a way to do this cheaply in assertion propagation.

[JosephTremoulet]

Dup of #7312

Sure is, thanks; closing.