Consolidate reduce_and_compute code

Consolidate reduce_and_compute code

This patch reshuffles the reduce_and_compute code so that the
structure matches other code structures of the same type elsewhere in
s_sin.c and s_sincos.c.  This is the beginning of an attempt to
consolidate and reduce code duplication in functions in s_sin.c to
make it easier to read and possibly also easier for the compiler to
optimize.

	* sysdeps/ieee754/dbl-64/s_sin.c (reduce_and_compute):
	Consolidate switch cases 0 and 2.