SVE intrinsics: Add constant folding for svindex.

This patch folds svindex with constant arguments into a vector series.
We implemented this in svindex_impl::fold using the function build_vec_series.
For example,
svuint64_t f1 ()
{
  return svindex_u642 (10, 3);
}
compiled with -O2 -march=armv8.2-a+sve, is folded to {10, 13, 16, ...}
in the gimple pass lower.
This optimization benefits cases where svindex is used in combination with
other gimple-level optimizations.
For example,
svuint64_t f2 ()
{
    return svmul_x (svptrue_b64 (), svindex_u64 (10, 3), 5);
}
has previously been compiled to
f2:
        index   z0.d, #10, #3
        mul     z0.d, z0.d, #5
        ret
Now, it is compiled to
f2:
        mov     x0, 50
        index   z0.d, x0, #15
        ret

We added test cases checking
- the application of the transform during gimple for constant arguments,
- the interaction with another gimple-level optimization.

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
	(svindex_impl::fold): Add constant folding.

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

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