SVE intrinsics: Fold constant operands for svlsl.

SVE intrinsics: Fold constant operands for svlsl.

This patch implements constant folding for svlsl. Test cases have been added to
check for the following cases:

Zero, merge, and don't care predication.
Shift by 0.
Shift by register width.
Overflow shift on signed and unsigned integers.
Shift on a negative integer.
Maximum possible shift, eg. shift by 7 on an 8-bit integer.

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

Signed-off-by: Soumya AR <soumyaa@nvidia.com>
gcc/ChangeLog:

	* config/aarch64/aarch64-sve-builtins-base.cc (svlsl_impl::fold):
	Try constant folding.
	* config/aarch64/aarch64-sve-builtins.cc (aarch64_const_binop):
	Return 0 if shift is out of range.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/sve/const_fold_lsl_1.c: New test.