Implement forward substitution utility to create larger trees for morph

[russellhadley]

Extend RyuJIT to use forward substitution to expand the impact of morph optimizations.

category:cq
theme:optimization
skill-level:expert
cost:extra-large

[russellhadley]

@erozenfeld for context.

[AndyAyersMS]

Also see #4655

[fiigii]

@AndyAyersMS Do you have any plan to add forward substitution? That would very good to optimize slow-LEA on x86/x64 architectures.

[AndyAyersMS]

We are starting to form up plans for 2.2. I am trying to get a handle on all the issues already open, and as part of this have been trying to cross-link similar things.

If you have examples in mind where we should generate different code, please make sure they are added to the relevant issues.