Similar idea to the combine.c and gcse.c patches.

2019-09-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* loop-iv.c: Include regs.h and function-abi.h.
	(simplify_using_initial_values): Use insn_callee_abi to get the
	ABI of the call insn target.  Conservatively assume that
	partially-clobbered registers are altered.

From-SVN: r276326