Add support for AArch64 system register names IP0, IP1, FP and LR.

	* config/tc-aarch64.c (reg_entry_reg_names): Add IP0,
	IP1, FP, and LR as register aliases of register 16, 17, 29
	and 30 respectively.
	* testsuite/gas/aarch64/diagnostic.l: Remove diagnostic
	prohibiting register 'lr' which is now an alias.
	* testsuite/gas/aarch64/diagnostic.s: Remove instruction
	utilizing register 'lr' which is now an alias.