SVE intrinsics: Fold constant operands for svmul.

This patch implements constant folding for svmul by calling
gimple_folder::fold_const_binary with tree_code MULT_EXPR.
Tests were added to check the produced assembly for different
predicates, signed and unsigned integers, and the svmul_n_* case.

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

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

gcc/
	* config/aarch64/aarch64-sve-builtins-base.cc (svmul_impl::fold):
	Try constant folding.

gcc/testsuite/
	* gcc.target/aarch64/sve/const_fold_mul_1.c: New test.