xtensa: Eliminate [DS]Cmode hard register clobber that is immediately followed...

xtensa: Eliminate [DS]Cmode hard register clobber that is immediately followed by whole overwrite the register

RTL expansion of substitution to [DS]Cmode hard register includes obstructive
register clobber.

A simplest example:

    double _Complex test(double _Complex c) {
      return c;
    }

will be converted to:

    (set (reg:DF 42 [ c ]) (reg:DF 2 a2))
    (set (reg:DF 43 [ c+8 ]) (reg:DF 4 a4))
    (clobber (reg:DC 2 a2))
    (set (reg:DF 2 a2) (reg:DF 42 [ c ]))
    (set (reg:DF 4 a4) (reg:DF 43 [ c+8 ]))
    (use (reg:DC 2 a2))
    (return)

and then finally:

    test:
	mov	a8, a2
	mov	a9, a3
	mov	a6, a4
	mov	a7, a5
	mov	a2, a8
	mov	a3, a9
	mov	a4, a6
	mov	a5, a7
	ret

As you see, it is so ridiculous.

This patch eliminates such clobber in order to prune away the wasted move
instructions by the optimizer:

    test:
	ret

gcc/ChangeLog:

	* config/xtensa/xtensa.md (DSC): New split pattern and mode iterator.