Make sure `case .. of True => ... | False => ...` compiles to `If ... ... ...`

[myreen]

Currently, cexp at source level doesn't have If. We need to make sure that the case expressions that really are If compile to If in CakeML.

[myreen]

Ah, @mn200 points out that If does exist but as a primitive op.

[mn200]

But not available to cexp programs, as those primitive ops are for exp only.