Add, rewrite, and/or reformat many comments.

Stop passing ostreams around: we already have one perfectly good ostream
and we can all share it.

Stop stashing a pointer to TargetData in the Pass object, because that will
lead to a crash if there are no functions in the module (ouch!)  Instead,
use addRequired() and getAnalysis(), like we always should have done.

Move the check for ConstantExpr up before the check for isPrimitiveType,
because we need to be able to catch e.g. ubyte (cast bool false to ubyte),
whose type is primitive but which is nevertheless a ConstantExpr, by calling
our specialized handler instead of the AsmWriter. This would result in
assembler errors when we would try to output something like ".byte (cast
bool false to ubyte)".

GC some unused variable declarations.

llvm-svn: 7265