SVE intrinsics: Refactor const_binop to allow constant folding of intrinsics.

This patch sets the stage for constant folding of binary operations for SVE
intrinsics:
In fold-const.cc, the code for folding vector constants was moved from
const_binop to a new function vector_const_binop. This function takes a
function pointer as argument specifying how to fold the vector elements.
The intention is to call vector_const_binop from the backend with an
aarch64-specific callback function.
The code in const_binop for folding operations where the first operand is a
vector constant and the second argument is an integer constant was also moved
into vector_const_binop to to allow folding of binary SVE intrinsics where
the second operand is an integer (_n).
To allow calling poly_int_binop from the backend, the latter was made public.

The patch was bootstrapped and regtested on aarch64-linux-gnu, no regression.
OK for mainline?

Signed-off-by: Jennifer Schmitz <jschmitz@nvidia.com>

gcc/
	* fold-const.h: Declare vector_const_binop.
	* fold-const.cc (const_binop): Remove cases for vector constants.
	(vector_const_binop): New function that folds vector constants
	element-wise.
	(int_const_binop): Remove call to wide_int_binop.
	(poly_int_binop): Add call to wide_int_binop.