[RISCV] Use getNaturalPointeeTypeAlignment to get alignment for stores created for vector builtins.

Instead of using scalar size divided by 8 for segment loads, get
the alignment from clang's type system.

Make vleff match for consistency.

Also replace uses of getPointerElementType() which will be removed as part of the OpaquePtr changes.

Reviewed By: HsiangKai

Differential Revision: https://reviews.llvm.org/D106738