[clang] [llvm] [RISCV] Add Spacemit X100 processor definition (PR #173988)

[Luke Lau via cfe-commits]

lukel97 wrote:

> The throughput of simple integer RVV instructions for LMUL=1 and LMUL<1 is the same, so you'd be wasting half of the performance defaulting to LMUL=½.

To clarify the loop vectorizer will still use LMUL 2 by default, and the SLP vectorizer will use whatever LMUL needed for a given vectorization tree. TuneDLenFactor2 mostly influences the decision to e.g. auto-vectorize code vs leaving it as scalar

https://github.com/llvm/llvm-project/pull/173988