[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