rtl-optimization/118662 - wrong combination of vector sign-extends

rtl-optimization/118662 - wrong combination of vector sign-extends

The following fixes an issue in the RTL combiner where we correctly
combine two vector sign-extends with a vector load

Trying 7, 9 -> 10:
    7: r106:V4QI=[r119:DI]
      REG_DEAD r119:DI
    9: r108:V4HI=sign_extend(vec_select(r106:V4QI#0,parallel))
   10: r109:V4SI=sign_extend(vec_select(r108:V4HI#0,parallel))
      REG_DEAD r108:V4HI

to

modifying insn i2     9: r109:V4SI=sign_extend([r119:DI])

but since r106 is used we wrongly materialize it using a subreg:

modifying insn i3    10: r106:V4QI=r109:V4SI#0

which of course does not work for modes with more than one component,
specifically vector and complex modes.

	PR rtl-optimization/118662
	* combine.cc (try_combine): When re-materializing a load
	from an extended reg by a lowpart subreg make sure we're
	not dealing with vector or complex modes.

	* gcc.dg/torture/pr118662.c: New testcase.