[PowerPC] Support symbolic u16imm operands

Currently, all instructions taking s16imm operands support symbolic
operands.  However, for u16imm operands, we only support actual
immediate integers.  This causes the assembler to reject code like

  ori %r5, %r5, symbol@l

This patch changes the u16imm operand definition to likewise
accept symbolic operands.  In fact, s16imm and u16imm can
share the same encoding routine, now renamed to getImm16Encoding.
Remove unused header.
[PowerPC] Accept 17-bit signed immediates for addis

The assembler currently strictly verifies that immediates for
s16imm operands are in range (-32768 ... 32767).  This matches
the behaviour of the GNU assembler, with one exception: gas
allows, as a special case, operands in an extended range
(-65536 .. 65535) for the addis instruction only (and its
extended mnemonic lis).

The main reason for this seems to be to allow using unsigned
16-bit operands for lis, e.g. like lis %r1, 0xfedc.

Since this has been supported by gas for a long time, and
assembler source code seen "in the wild" actually exploits
this feature, this patch adds equivalent support to LLVM
for compatibility reasons.