JIT: Transform 'SELECT(cond, cns, cns)' to 'cns'

[BoyBaykiller]

Example:

static bool IsDefaultValue(double value)
{
    return 0.0.Equals(value);
}

SELECT created:
               [000076] -----------                         *  SELECT    int   
N003 (  5,  8) [000056] J----+-N---                         +--*  EQ        int    $141
N001 (  1,  2) [000034] -----+-----                         |  +--*  LCL_VAR   double V00 arg0         u:1 $80
N002 (  1,  2) [000055] -----+-----                         |  \--*  LCL_VAR   double V00 arg0         u:1 (last use) $80
N001 (  1,  1) [000041] -----+-----                         +--*  CNS_INT   int    0 $40
N001 (  1,  1) [000075] -----------                         \--*  CNS_INT   int    0 $40

Note: gtFoldExprConditional can also handle this...

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[tannergooding]

Why here instead of gtFoldExpr so that we avoid ending up with such conditional select nodes in the first place?

[BoyBaykiller]

I am not sure how I am supposed to improve gtFoldExpr to handle this.
We enter if-conversion with IR equivalent to:

if (a == b)
{
    result = false;
}
else
{
    result = false;
}

return result;

where a and b are of type double so can't be folded. I am folding based on the true/false paths storing the same values, but that first requires the necessary control flow simplification which if conversion does to collapse this into a single SELECT which can then be analysed. Doesn't look like a gtFoldExpr job to me. Although there might be some opportunity very early on. Anyway I think we should have this either way.