append flags of subclasses to the unused bits of bitfields in Expression

[rainers]

reduces memory used in derived classes, including padding bytes after flags

A minor downside is that non-zero initialization has to be moved to the ctor. For expressions this only occured once.

[github-actions]

DMD perf check

Metric	Base	PR	delta
compile hello.d (instr)	215.9 M	215.9 M	-0.03%
compile hello.d -O (instr)	234.5 M	234.4 M	-0.02%
compile Phobos (instr)	5,077.4 M	5,079.0 M	+0.03%
compile Phobos codegen (instr)	1,453.3 M	1,455.0 M	+0.12%
compile vibe.d (instr)	15,136.3 M	15,138.8 M	+0.02%
dmd binary size (stripped)	6.92 MB	6.92 MB	-0.06%
hello binary size	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	44 MB	43 MB	-0.30%
peak RSS (compile Phobos)	631 MB	631 MB	-0.07%
peak RSS (compile vibe.d)	1980 MB	1977 MB	-0.16%

[rainers]

Improvements are a bit disappointing, probably because most class instance sizes don't jump into the previous 16-byte category used by bump-pointer-allocation.