Remove unused IrTypeFunction::fty().

The design split between IrFuncTy/IrTypeFunction might need
a review at some point.
Initialize IrTypeAggr members in order to silence warning.
Finally fix the _d_invariant declaration on x86.
Complete IrStruct->IrAggr rename.
Make -release only disable bounds checking in non-@safe code.
Fold in broken std.exception test case fix.