Rollup merge of #127168 - DianQK:cast-size, r=workingjubilee

Use the aligned size for alloca at args/ret when the pass mode is cast

Fixes #75839. Fixes #121028.

The `load` and `store` instructions in LLVM access the aligned size. For example, `load { i64, i32 }` accesses 16 bytes on x86_64: https://alive2.llvm.org/ce/z/n8CHAp.

BTW, this example is expected to be optimized to immediate UB by Alive2: https://rust.godbolt.org/z/b7xK7hv1c and https://alive2.llvm.org/ce/z/vZDtZH.

r? compiler

Author: matthiaskrgr