[AArch64] Fix ICEs in aarch64_print_operand

[AArch64] Fix ICEs in aarch64_print_operand

Three related regression fixes:

- We can't use asserts like:

    gcc_assert (GET_MODE_SIZE (mode) == 16);

  in aarch64_print_operand because it could trigger for invalid user input.

- The output_operand_lossage in aarch64_print_address_internal:

    output_operand_lossage ("invalid operand for '%%%c'", op);

  wasn't right because "op" is an rtx_code enum rather than the
  prefix character.

- aarch64_print_operand_address shouldn't call output_operand_lossage
  (because it doesn't have a prefix code) but instead fall back to
  output_addr_const.

2017-12-05  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* config/aarch64/aarch64.c (aarch64_print_address_internal): Return
	a bool success value.  Don't call output_operand_lossage here.
	(aarch64_print_ldpstp_address): Return a bool success value.
	(aarch64_print_operand_address): Call output_addr_const if
	aarch64_print_address_internal fails.
	(aarch64_print_operand): Don't assert that the mode is 16 bytes for
	'y'; call output_operand_lossage instead.  Call output_operand_lossage
	if aarch64_print_ldpstp_address fails.

gcc/testsuite/
	* gcc.target/aarch64/asm-2.c: New test.
	* gcc.target/aarch64/asm-3.c: Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@255481 138bc75d-0d04-0410-961f-82ee72b054a4