Use operand->extract to provide defaults for optional PowerPC operands

Use operand->extract to provide defaults for optional PowerPC operands

Most optional operands to powerpc instructions use a default value of
zero, but there are a few exceptions.  Those have been handled by
PPC_OPERAND_OPTIONAL_VALUE and an entry in the powerpc_operands table
for the default value, smuggled in the shift field.  This patch
changes that to using the operand extract function to provide non-zero
defaults.

I've also moved the code determining whether optional operands are
provided or omitted, to the point the first optional operand is seen,
and allowed for the possibility of optional base register operands
in a future patch.

The patch does change the error you get on invalid assembly like

  ld 3,4

You'll now see "missing operand" rather than
"syntax error; end of line, expected `('".

gas/
	* config/tc-ppc.c (md_assemble): Delay counting of optional
	operands until one is encountered.  Allow for the possibility
	of optional base regs, ie. PPC_OPERAND_PARENS.  Call
	ppc_optional_operand_value with extra args.
include/
	* opcode/ppc.h (struct powerpc_operand): Correct "insert" comment.
	Mention use of "extract" function to provide default value.
	(PPC_OPERAND_OPTIONAL_VALUE): Delete.
	(ppc_optional_operand_value): Rewrite to use extract function.
opcodes/
	* ppc-dis.c (operand_value_powerpc): Init "invalid".
	(skip_optional_operands): Count optional operands, and update
	ppc_optional_operand_value call.
	* ppc-opc.c (extract_dxdn): Remove ATTRIBUTE_UNUSED from used arg.
	(extract_vlensi): Likewise.
	(extract_fxm): Return default value for missing optional operand.
	(extract_ls, extract_raq, extract_tbr): Likewise.
	(insert_sxl, extract_sxl): New functions.
	(insert_esync, extract_esync): Remove Power9 handling and simplify.
	(powerpc_operands <FXM4, TBR>): Delete PPC_OPERAND_OPTIONAL_VALUE
	flag and extra entry.
	(powerpc_operands <SXL>): Likewise, and use insert_sxl and
	extract_sxl.