simplify-rtx: Simplify sign_extend of lshiftrt to zero_extend (PR68330)

simplify-rtx: Simplify sign_extend of lshiftrt to zero_extend (PR68330)

Since r230164, in PR68330 combine ends up with a sign_extend of an
lshiftrt by some constant, and it does not know to morph that into a
zero_extract (the extend will always extend with zeroes).  I think
it is best to let simplify-rtx always replace such a sign_extend by
a zero_extend, after which everything works as expected.

2015-11-15  Segher Boessenkool  <segher@kernel.crashing.org>

	PR rtl-optimization/68330
	* simplify-rtx.c (simplify_unary_operation_1): Simplify SIGN_EXTEND
	of LSHIFTRT by a non-zero constant integer.

From-SVN: r230429