Hide call_used_regs in target-independent code

Hide call_used_regs in target-independent code

Now that tests of call_used_regs go through call_used_or_fixed_reg_p,
we can hide call_used_regs from target-independent code.  (It still
needs to be available to targets for the conditional register usage
hooks.)

2019-09-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* hard-reg-set.h (call_used_regs): Only define if IN_TARGET_CODE.
	(call_used_or_fixed_reg_p): Expand definition of call_used_regs.
	* reginfo.c (call_used_regs): New macro.

From-SVN: r275604