[1/11][IR] Permit load/store/alloca for struct of the same scalable vector type

[1/11][IR] Permit load/store/alloca for struct of the same scalable vector type

This patch-set aims to simplify the existing RVV segment load/store
intrinsics to use a type that represents a tuple of vectors instead.

To achieve this, first we need to relax the current limitation for an
aggregate type to be a target of load/store/alloca when the aggregate
type contains homogeneous scalable vector types. Then to adjust the
prolog of an LLVM function during lowering to clang. Finally we
re-define the RVV segment load/store intrinsics to use the tuple types.

The pull request under the RVV intrinsic specification is
riscv-non-isa/rvv-intrinsic-doc#198

---

This is the 1st patch of the patch-set. This patch is originated from
D98169.

This patch allows aggregate type (StructType) that contains homogeneous
scalable vector types to be a target of load/store/alloca. The RFC of
this patch was posted in LLVM Discourse.

https://discourse.llvm.org/t/rfc-ir-permit-load-store-alloca-for-struct-of-the-same-scalable-vector-type/69527

The main changes in this patch are:

Extend `StructLayout::StructSize` from `uint64_t` to `TypeSize` to
accommodate an expression of scalable size.

Allow `StructType:isSized` to also return true for homogeneous
scalable vector types.

Let `Type::isScalableTy` return true when `Type` is `StructType`
and contains scalable vectors

Extra description is added in the LLVM Language Reference Manual on the
relaxation of this patch.

Authored-by: Hsiangkai Wang <kai.wang@sifive.com>
Co-Authored-by: eop Chen <eop.chen@sifive.com>
Reviewed By: craig.topper, nikic

Differential Revision: https://reviews.llvm.org/D146872