[RISCV] Fix stack slot for argument types (Bug 49500)

This is an complementary/alternative fix for D99068. It takes a slightly
different approach by explicitly summing up all of the required split
part type sizes and ensuring we allocate enough space for them. It also
takes the maximum alignment of each part.

Compared with D99068 there are fewer changes to the stack objects in
existing tests. However, @luismarques has shown in that patch that there
are opportunities to reduce our stack usage in the future.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D99087