[arm] Improve constant handling for subvsi4.

[arm] Improve constant handling for subvsi4.

This patch addresses constant handling in subvsi4.  Either operand may
be a constant.  If the second input (operand[2]) is a constant, then
we can canonicalize this into an addition form, providing we take care
of the INT_MIN case.  In that case the negation has to handle the fact
that -INT_MIN is still INT_MIN and we need to ensure that a subtract
operation is performed rather than an addition.  The remaining cases
are largely duals of the usubvsi4 expansion.

This patch also fixes a technical correctness bug in the old
expansion, where we did not realy describe the test for overflow in
the RTL.  We seem to have got away with that, however...

	* config/arm/arm.md (subv<mode>4): Delete.
	(subvdi4): New expander pattern.
	(subvsi4): Likewise.  Handle some immediate values.
	(subvsi3_intmin): New insn pattern.
	(subvsi3): Likewise.
	(subvsi3_imm1): Likewise.
	* config/arm/arm.c (select_cc_mode): Also allow minus for CC_V
	idioms.

From-SVN: r277189