xtensa: Refurbish xtensa_legitimize_address()

The successive modifications made to 'xtensa_legitimize_address()' have
worsened the readability of the code, and the modifications themselves
are difficult to understand.

Therefore, this patch resolves these issues by completely rewriting the
function to make its intent clear.

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_legitimize_address):
	Completely rewrite it into an equivalent but more easily under-
	standable description.