JIT: Improve strength reduction's prediction of loop reversal (#104800)

The current goal is to make sure strength reduction does not introduce new
primary IVs. To do so we rely on the downwards-loop transformation to get rid of
uses of the primary IV in the loop test.
This PR refines the checks that strength reduction was using to predict whether
a use of the primary IV in a loop test will be removed. It should now be the
exact same checks as the downwards-transformation uses.

Additionally, it means we can remove `CursorInfo::IsInsideExitTest`; we now just
avoid creating the cursor in the first place if we know the use will be
removable.

Author: jakobbotsch

src/coreclr/jit/compiler.h

@@ -7646,6 +7646,8 @@ class Compiler
                                          FlowGraphNaturalLoop*   loop,
                                          BasicBlock*             exiting,
                                          LoopLocalOccurrences*   loopLocals);
+    bool optCanAndShouldChangeExitTest(GenTree* cond, bool dump);
+    bool optPrimaryIVHasNonLoopUses(unsigned lclNum, FlowGraphNaturalLoop* loop, LoopLocalOccurrences* loopLocals);
     bool optWidenPrimaryIV(FlowGraphNaturalLoop* loop,
                            unsigned              lclNum,
                            ScevAddRec*           addRec,