[arm] Add alternative canonicalizations for subtract-with-carry + shift

[arm] Add alternative canonicalizations for subtract-with-carry + shift

This patch adds a couple of alternative canonicalizations to allow
combine to match a subtract-with-carry operation when one of the operands
is shifted first.  The most common case of this is when combining a
sign-extend of one operand with a long-long value during subtraction.
The RSC variant is only enabled for Arm, the SBC variant for any 32-bit
compilation.

	* config/arm/arm.md (subsi3_carryin_shift_alt): New pattern.
	(rsbsi3_carryin_shift_alt): Likewise.

From-SVN: r277176