[tree-ssa-math-opts] Expand pow (x, CONST) using square roots when possible

[tree-ssa-math-opts] Expand pow (x, CONST) using square roots when possible

	* params.def (PARAM_MAX_POW_SQRT_DEPTH): New param.
	* tree-ssa-math-opts.c: Include params.h
	(pow_synth_sqrt_info): New struct.
	(representable_as_half_series_p): New function.
	(get_fn_chain): Likewise.
	(print_nested_fn): Likewise.
	(dump_fractional_sqrt_sequence): Likewise.
	(dump_integer_part): Likewise.
	(expand_pow_as_sqrts): Likewise.
	(gimple_expand_builtin_pow): Use above to attempt to expand
	pow as series of square roots.  Removed now unused variables.

	* gcc.target/aarch64/pow-sqrt-synth-1.c: New test.
	* gcc.dg/pow-sqrt.x: New file.
	* gcc.dg/pow-sqrt-1.c: New test.
	* gcc.dg/pow-sqrt-2.c: Likewise.
	* gcc.dg/pow-sqrt-3.c: Likewise.

From-SVN: r223167