Add salvageLoadDebugInfo to handle LoadInst
Before deleting a load, simply rewrite its debug info to refer to the
loaded address:
%l = load %a
debug_value %l, loc0, scope0, var0
--->
debug_value %a, loc0, scope0, var0, expr op_deref
%l = load %a
Note that alloc_stack addresses do not require the addition of
op_deref because they are already special-cased in IRGen.
This will be called from salvageDebugInfo when it is supported by
optmizations. Until then, it is only called selectively at -Onone.
