Constbits: Implemented precise transfer function for Add, Sub and Minus.

This accidentally fixes opt/fehler236.c.
Generalized -a & 1 => a & 1 optimization.
Added generalized optimization of -a | 0xF..FE => a | 0xF..FE.
Use tarval_shl_unsigned.
try to protect users from adding spills/stores to the frame entity coalescer (only loads may be added)
only report loads to spillslot coalescer
normalize modify_flags, add missing flags
IncSP needs modify_flags bit set
cleanup
fix warning
scalar_replace: cleanup, small improvements

- Does not invalidate graph properties anymore if nothing changed
- Only produce Bad/Jmp Tuple predecessors for possible throwing
  Load/Store nodes
- Improve check_load_store() logic slightly and rename to
  is_conv_bitcast()
scalar_replace: bail out if structs are cast directly to load store and through Sel

This fixes gcc/pr36988.c
constbits: Normalize style.
ir: Remove redundant if (x) before set_Block_entity(b, x);

It's safe to pass a null pointer to set_Block_entity().
beirgmod: Remove unused attribute irg from struct remove_dead_nodes_env_t.
scalar_replace: load/stores on child sels are no problem