[RFC, PowerPC, Darwin] Clean up generation of ha16() / lo16() markers

[Ulrich Weigand]

Hello,

when targeting the Darwin assembler, we need to generate markers ha16() and
lo16() to designate the high and low parts of a (symbolic) immediate.  This
is necessary not just for plain symbols, but also for certain symbolic
expression, typically along the lines of ha16(A - B).  The latter doesn't
work when simply using VariantKind flags on the symbol reference.

This is why the current back-end used hacks (explicitly called out as such
via multiple FIXMEs) in the symbolLo/symbolHi print methods.  I'd like to
get rid of this special treatment as a first step towards getting rid of
symbolLo/symbolHi itself.  This is interesting because it will allow
non-standard (but legal) use of the @l and @hi markers, e.g. to do
"lis ..., symbol at l"  (which is valid if unusual assembler, but will not be
handled correctly by the llvm assembler right now).

The attached patch uses target-defined MCExpr codes to represent the Darwin
ha16/lo16 constructs, following along the lines of the equivalent solution
used by the ARM back end to handle their :upper16: / :lower16: markers.
This allows us to get rid of special handling both in the symbolLo/symbolHi
print method and in the common code MCExpr::print routine.  Instead, the
ha16 / lo16 markers are printed simply in a custom print routine for the
target MCExpr types.

I don't have a live Darwin system to test on, but the Darwin assembler
tests in the test suite all still pass.

Does this look reasonable?

Bye,
Ulrich

(See attached file: diff-reloc-mcexpr)
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