DebugInfo: Don't lose unreferenced non-trivial by-value parameters

A FastISel optimization was causing us to emit no information for such
parameters & when they go missing we end up emitting a different
function type. By avoiding that shortcut we not only get types correct
(very important) but also location information (handy) - even if it's
only live at the start of a function & may be clobbered later.

Reviewed/discussion by Evan Cheng & Dan Gohman.