[SveEmitter] Fix encoding/decoding of SVETypeFlags

Summary:
This issue was introduced when reworking D75861. The bug isn't
actually hit with current unit tests because the contiguous loads/stores
infer the EltType and the MemEltType from the pointer and result, rather
than using the flags. But it will be needed for other intrinsics, such as
gather/scatter.

Reviewers: SjoerdMeijer, Andrzej

Reviewed By: SjoerdMeijer

Subscribers: andwar, tschuett, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76617