[arm] Allow the summation result of signed add-with-overflow to be discarded.

[arm] Allow the summation result of signed add-with-overflow to be discarded.

This patch matches the signed add-with-overflow patterns when the
summation itself is dropped.  In this case we can use CMN (or CMP with
some immediates).  There are a small number of constants in thumb2
where this can result in less dense code (as we lack 16-bit CMN with
immediate patterns).  To handle this we use peepholes to try these
alternatives when either a scratch is available (0 <= i <= 7) or the
original register is dead (0 <= i <= 255).  We don't use a scratch in
the pattern as if those conditions are not satisfied then the 32-bit
form is preferable to forcing a reload.

	* config/arm/arm.md (addsi3_compareV_reg_nosum): New insn.
	(addsi3_compareV_imm_nosum): New insn.  Also add peephole2 patterns
	to transform this back into the summation version when that leads
	to smaller code.

From-SVN: r277185