Further investigation showed that my previous patch for this issue was
still incomplete.

The problem stemmed from what I suspect was a mis-understanding of the
way overflow is calculated on aarch64 when values are subtracted (and
hence in comparisons).  In this case, unlike addition, the carry flag is
/cleared/ if there is overflow (technically, underflow) and set when
that does not happen.  This patch clears up this issue by using CCmode
for all subtractive operations (this can fully describe the normal
overflow conditions without anything particularly fancy); clears up the
way we express normal unsigned overflow using CC_Cmode (the result of a
sum is less than one of the operands) and adds a new mode, CC_ADCmode to
handle expressing overflow of an add-with-carry operation, where the
standard idiom is no-longer sufficient to describe the overflow condition.

	PR target/86891
	* config/aarch64/aarch64-modes.def: Add comment about how the carry
	bit is set by add and compare.
	(CC_ADC): New CC_MODE.
	* config/aarch64/aarch64.c (aarch64_select_cc_mode): Use variables
	to cache the code and mode of X.  Adjust the shape of a CC_Cmode
	comparison.  Add detection for CC_ADCmode.
	(aarch64_get_condition_code_1): Update code support for CC_Cmode.  Add
	CC_ADCmode.
	* config/aarch64/aarch64.md (uaddv<mode>4): Use LTU with CCmode.
	(uaddvti4): Comparison result is in CC_ADCmode and the condition is GEU.
	(add<mode>3_compareC_cconly_imm): Delete.  Merge into...
	(add<mode>3_compareC_cconly): ... this.  Restructure the comparison
	to eliminate the need for zero-extending the operands.
	(add<mode>3_compareC_imm): Delete.  Merge into ...
	(add<mode>3_compareC): ... this.  Restructure the comparison to
	eliminate the need for zero-extending the operands.
	(add<mode>3_carryin): Use LTU for the overflow detection.
	(add<mode>3_carryinC): Use CC_ADCmode for the result of the carry out.
	Reexpress comparison for overflow.
	(add<mode>3_carryinC_zero): Update for change to add<mode>3_carryinC.
	(add<mode>3_carryinC): Likewise.
	(add<mode>3_carryinV): Use LTU for carry between partials.
	* config/aarch64/predicates.md (aarch64_carry_operation): Update
	handling of CC_Cmode and add CC_ADCmode.
	(aarch64_borrow_operation): Likewise.

Bootstrapped on aarch64-linux.  Applied to trunk.