On ARM, the SBC instruction is defined as

  Ra - Rb - ~C

where C is the carry flag.  But -Rb = ~Rb + 1, so this is equivalent to

  Ra + ~Rb + 1 - ~C

which then simplifies to

  Ra + ~Rb + C

which is essentially an add-with-carry with one operand inverted.  We
can define RTL patterns to match this.  In thumb2 we can only match
when the operands are both registers, but in Arm state we can also use
RSC to match when Rn is either a constant or a shifted operand.

This overall simplifies some cases of 64-bit arithmetic, for example, 

int64_t f (int64_t a, int64_t b) { return a + ~b; }

will now compile to

  MVN  R2, R2
  ADDS R0, R0, R2
  SBC  R1, R1, R3

	* config/arm/arm.md (add_not_cin): New insn.
	(add_not_shift_cin): Likewise.

From-SVN: r277676