[PowerPC] Fix hi/lo encoding in old-style code emitter

This patch implements the equivalent change to r182091/r182092
in the old-style code emitter.  Instead of having two separate
16-bit immediate encoding routines depending on the instruction,
this patch introduces a single encoder that checks the machine
operand flags to decide whether the low or high half of a
symbol address is required.

Since now both encoders make no further distinction between
"symbolLo" and "symbolHi", the .td operand can now use a
single getS16ImmEncoding method.

Tested by running the old-style JIT tests on 32-bit Linux.