Remove constant vec_select restriction.

Remove constant vec_select restriction.

The vec_select operator is documented to require a const_int for the lane
selector operand, but GCN has an instruction that can select the lane at
runtime, so it seems reasonable to remove this restriction.

This patch simply replaces assertions that the operand is constant with early
exits from the optimizers.  I think it's reasonable that vec_select with a
non-constant operand cannot be optimized, yet.

Also included is the necessary documentation tweak.

2018-09-19  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* doc/rtl.texi: Adjust vec_select description.
	* simplify-rtx.c (simplify_binary_operation_1): Allow VEC_SELECT to use
	non-constant selectors.

From-SVN: r264423