JIT: recognize profitable factoring of common commutative operands in if-conversionrecognize profitable factoring of common commutative operands in if-c…

[Dimoner]

This change improves optIfConversion by recognizing a subset of branch expressions that share common commutative operands and by using that information in the if-conversion profitability check and final expression construction.

Previously

public static int ForDumpAnalyse1(int x, int y, int z, int q)
    {
        if (x < y)
        {
            return x + z + 50 + q;
        }
        
        return x + y + 50 + q;
    }

create

G_M000_IG01:
            stp     fp, lr, [sp, #-0x10]!
            mov     fp, sp
 
G_M000_IG02:
            cmp     w0, w1
            blt     G_M000_IG05
 
G_M000_IG03:
            add     w0, w0, w1
            add     w0, w0, w3
            add     w0, w0, #50
 
G_M000_IG04:
            ldp     fp, lr, [sp], #0x10
            ret     lr
 
G_M000_IG05:
            add     w0, w0, w2
            add     w0, w0, w3
            add     w0, w0, #50
 
G_M000_IG06:
            ldp     fp, lr, [sp], #0x10
            ret     lr

and

 public static int ForDumpAnalyse1(int x, int y, int z, int q)
    {
        if (x < y)
        {
            return x + z + 50;
        }
        
        return x + y + 50;
    }

create

            add     w3, w0, w1
            add     w3, w3, #50
            add     w2, w0, w2
            add     w2, w2, #50
            cmp     w0, w1
            csel    w0, w3, w2, ge

For example, instead of selecting between the full expressions:
select(cond, x + z + 50, x + y + 50)
the JIT can produce a shape equivalent to:
(select(cond, z, y) + x) + 50

Now

public static int ForDumpAnalyse1(int x, int y, int z, int q)
    {
        if (x < y)
        {
            return x + z + 50 + q;
        }
        
        return x + y + 50 + q;
    }

result

            cmp     w0, w1
            csel    w1, w1, w2, ge
            add     w0, w1, w0
            add     w0, w0, w3
            add     w0, w0, #50

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[EgorBo]

NOTE: for changes with quite a few lines of code there must be a justification. Usually, we rely on SPMI reports (you can find one by clicking on runtime-coreclr superpmi-diffs -> View more details on Azure Pipelines -> Extensions. Which is this report in case of this PR (for the last change). So far, the diffs are too small to justify this number of risky changes (it also means the test coverage for this transformation is poor).