Mars assembly: No way to directly call constructors

[Matt Giuca]

There is no way for Mars assembly code to directly call a constructor function. Instead, it needs to do a ld_ctor (which creates a local variable with the constructor function) followed by a call (to the local variable). The same applies for parcall.

This is potentially quite inefficient on backends (which could potentially end up constructing a closure for the ld_ctor) and requires multi-instruction optimisation to fix. Therefore, add call_ctor and parcall_ctor instructions to directly call/parcall a constructor function, as parallels to call_global and parcall_global.

Note that this would then create a parallel of the situation in bug #766795, where ld_ctor is redundant for parcall_ctor with 0 arguments, and so ld_ctor should be removed. However, ld_ctor fills the constructor version of BOTH ld_func and ld_cgc -- only the function loading role is redundant. ld_ctor is still the only way to load a constructor constant symbol. Therefore, rename ld_ctor to ld_ctorsym, and change its semantics so it ONLY loads constructor constant symbols. For loading constructor functions, parcall_ctor should always be used.

[Matt Giuca (mgiuca)]

Fixed in no-ldfunc r1217; trunk r1215.