[AArch64] Fix cost of (plus ... (const_int -C))

The PLUS handling in aarch64_rtx_costs only checked for nonnegative
constants, meaning that simple immediate subtractions like:

  (set (reg R1) (plus (reg R2) (const_int -8)))

had a cost of two instructions.

2019-09-26  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_rtx_costs): Use
	aarch64_plus_immediate rather than aarch64_uimm12_shift
	to test for valid PLUS immediates.

From-SVN: r276140

gcc/ChangeLog

+2019-09-26  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* config/aarch64/aarch64.c (aarch64_rtx_costs): Use
+	aarch64_plus_immediate rather than aarch64_uimm12_shift
+	to test for valid PLUS immediates.
+
 2019-09-25  Martin Jambor  <mjambor@suse.cz>
 
 	* tree-sra.c (no_accesses_p): Remove.

gcc/config/aarch64/aarch64.c

@@ -10753,7 +10753,7 @@ cost_plus:
 	  }
 
 	if (GET_MODE_CLASS (mode) == MODE_INT
-	    && ((CONST_INT_P (op1) && aarch64_uimm12_shift (INTVAL (op1)))
+	    && (aarch64_plus_immediate (op1, mode)
 		|| aarch64_sve_addvl_addpl_immediate (op1, mode)))
 	  {
 	    *cost += rtx_cost (op0, mode, PLUS, 0, speed);