SI-6331 Avoid typing an If tree with a constant type.

The fast path in typedIf added in 8552740b avoided lubbing the if/else branch types
if they are identical, but this fails to deconst the type. This could lead to the entire
if expression being replaced by a constant.

Also introduces a new tool in partest for nicer checkfiles.

// in Test.scala
trace(if (t) -0d else 0d)

// in Test.check
trace> if (Test.this.t)
  -0.0
else
  0.0
res: Double = -0.0

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