JIT: experimental changes to redundant branch opts

Recognize a particular case where a control flow pattern involving two BBJ_COND
blocks with relops that test the same VNs can be simplified to a single relop
in the dominating block.

As part of this, teach VN about some of the rudiments of boolean simplification
(DeMorgan's laws)  and how to simplify some NOT / AND / OR expressions involving
relops.

Addresses some of the cases in dotnet#81220.

src/coreclr/jit/compiler.h

@@ -7142,6 +7142,8 @@ class Compiler
     PhaseStatus optRedundantBranches();
     bool optRedundantRelop(BasicBlock* const block);
     bool optRedundantBranch(BasicBlock* const block);
+    bool optHasIgnorableSideEffects(BasicBlock* const block);
+    bool optSubsumeRelop(BasicBlock* const block, BasicBlock* const domBlock, bool trueReaches);
     bool optJumpThreadDom(BasicBlock* const block, BasicBlock* const domBlock, bool domIsSameRelop);
     bool optJumpThreadPhi(BasicBlock* const block, GenTree* tree, ValueNum treeNormVN);
     bool optJumpThreadCheck(BasicBlock* const block, BasicBlock* const domBlock);