Deduce the mode automatically from the given entity.

This removes the strange entity/type duality and we get rid of union symconst_symbol.

Just use get_mode_name() instead.

Its only use is pointless.
The following call to FormatFirmObject() detects the same problem.

Just one variable to hold them is sufficient.

This fixes backend/sparc_shr.c.

This fixes backend/sparc_cmp.c.

This reverts commit f56426a9.
Skipping a bitfields within a struct leads to wrong offsets.

This avoids problems where fixing Sels creates new Projs which must not
be changed. The previously broke valid cases in opt/structcall3.c for sparc.

Previously only valid identifier were accepted.

This fixes opt/fehler226.c.

Also adds accompanying bitset_t functions and unittests.

This API function is now also visible for the optimize variant.

This fixes opt/fehler225.c

This fixes opt/dwlower3.c.

This fixes opt/fehler223.c.