Fix ivopts estimates for internal functions

tree-ssa-loop-ivopts.c:loop_body_includes_call was treating internal
calls such as IFN_SQRT as clobbering all caller-saved registers, which
I don't think is appropriate for any current internal function.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

gcc/
	* tree-ssa-loop-ivopts.c (loop_body_includes_call): Don't assume
	that internal functions will clobber all caller-saved registers.

From-SVN: r236780