Fixes dotnet#123858.

This PR improves JIT codegen for multi-target `switch`-style tests by
converting eligible cases into branchless checks. The goal is to produce
optimal codegen that matches the intent of C# pattern matching with `or`
(e.g., `x is A or B or C`).

When a switch has exactly 2 unique successors (all non-default cases
target one block, default targets another), convert it from Switch to an
unsigned range comparison (In `fgOptimizeSwitchBranches`).
When both targets are simple return blocks, `fgFoldCondToReturnBlock`
further folds this into branchless return.

## Example
```csharp
        private static bool IsLetterCategory(UnicodeCategory uc)
        {
            return uc == UnicodeCategory.UppercaseLetter
                 || uc == UnicodeCategory.LowercaseLetter
                 || uc == UnicodeCategory.TitlecaseLetter
                 || uc == UnicodeCategory.ModifierLetter
                 || uc == UnicodeCategory.OtherLetter;
        }
```

## Before
```asm
cmp      ecx, 4
ja       SHORT G_M22758_IG05
mov      eax, 1
ret
G_M22758_IG05:
xor      eax, eax
ret
```

## After
```asm
cmp      ecx, 4
setbe    al
movzx    rax, al
ret
```


## Details

- The comparison direction is chosen to GT_LT. 
- Edge dup counts are fixed up after conversion.
- Added tests for zero-based and non-zero-based consecutive ranges.

## ASMDiffs

- SPMI asmdiffs show code size improvements of 83 bytes.

---------

Co-authored-by: Egor Bogatov <egorbo@gmail.com>