llvm.sadd.with.overflow: vector types not lowered

Currently `SPIRVLowerLLVMIntrinsic.cpp` maps `llvm.sadd.with.overflow` for scalar types `i8`/`i16`/`i32`/`i64` only (the `i8` entry was added by #3673).

Vector variants are entirely absent from the map, so any LLVM IR using e.g. `llvm.sadd.with.overflow.v2i32` fails to translate.
SPIR-V supports integer vectors of 2, 3, 4, 8, and 16 components, leaving 20 vector variants uncovered: `v{2,3,4,8,16}i{8,16,32,64}`.

The proper fix, as noted in the review of #3673, is to replace the hand-authored IR strings in `LLVMSaddWithOverflow.h` with a programmatic IR expansion pass that generates the lowering for every valid type at translation time.

Start by reading SPIRVLowerLLVMIntrinsic.cpp and the hand-authored IR in LLVMSaddWithOverflow.h, then review the approach discussed in #3673. Determine how a programmatic IR expansion can cover the valid scalar and vector types at translation time. Done means all v{2,3,4,8,16}i{8,16,32,64} variants of llvm.sadd.with.overflow translate successfully.

Written by the indexing model from the issue text.