[AArch64] Properly handle SHIFT ops and EXTEND in aarch64_rtx_mult_cost

[AArch64] Properly handle SHIFT ops and EXTEND in aarch64_rtx_mult_cost

	* config/aarch64/aarch64.c (aarch64_shift_p): New function.
	(aarch64_rtx_mult_cost): Update comment to reflect that it also handles
	combined arithmetic-shift ops.  Properly handle all shift and extend
	operations that can occur in combination with PLUS/MINUS.
	Rename maybe_fma to compound_p.
	(aarch64_rtx_costs): Use aarch64_shift_p when costing compound
	arithmetic and shift operations.

From-SVN: r222624