Accept code attributes as rtx codes in .md files

Accept code attributes as rtx codes in .md files

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