This is a major cleanup of the instruction metadata interfaces that

I asked Devang to do back on Sep 27.  Instead of going through the
MetadataContext class with methods like getMD() and getMDs(), just
ask the instruction directly for its metadata with getMetadata()
and getAllMetadata().

This includes a variety of other fixes and improvements: previously
all Value*'s were bloated because the HasMetadata bit was thrown into
value, adding a 9th bit to a byte.  Now this is properly sunk down to
the Instruction class (the only place where it makes sense) and it
will be folded away somewhere soon.

This also fixes some confusion in getMDs and its clients about 
whether the returned list is indexed by the MDID or densely packed.
This is now returned sorted and densely packed and the comments make
this clear.

This introduces a number of fixme's which I'll follow up on.

llvm-svn: 92235