The recent AArch64 absolute difference patterns had to go through
some hoops to pair max/min rtx codes with the same signedness.
I also need to pair signed/unsigned codes with sign/zero extension
for some SVE ACLE patterns.

This patch therefore supports <...> as rtx codes, like we already
do for modes.

2019-05-12  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* doc/md.texi: Document use of code attributes in rtx patterns.
	* read-md.h (rtx_reader::rtx_alloc_for_name): New member function.
	* read-rtl.c (find_code): Split out search loops into...
	(maybe_find_code): ...this new function.
	(check_code_iterator): Make the error message more informative.
	(check_code_attribute): New function.
	(rtx_reader::rtx_alloc_for_name): Likewise.
	(rtx_reader::read_rtx_code): Use rtx_alloc_for_name.
	* config/aarch64/predicates.md (aarch64_smin, aarch64_umin): Delete.
	* config/aarch64/aarch64-simd.md (*aarch64_<su>abd<mode>_3): Use
	<max_opp> directly as an rtx code instead of via a match_operator.
	* config/aarch64/aarch64-sve.md (aarch64_<su>abd<mode>_3): Likewise.
	(<su>abd<mode>_3): Update accordingly.

From-SVN: r271107