[llvm] [RISCV] Support postRA vsetvl insertion pass (PR #70549)

[Luke Lau via llvm-commits]

================
@@ -96,6 +96,10 @@ static cl::opt<bool> EnableMISchedLoadClustering(
     cl::desc("Enable load clustering in the machine scheduler"),
     cl::init(false));
 
+static cl::opt<bool> EnableVSETVLIAfterRVVRegAlloc(
+    "riscv-vsetvli-after-rvv-regalloc", cl::Hidden,
----------------
lukel97 wrote:

Nit, should probably just use `vsetvl` to be consistent with the other CLI option `-riscv-disable-insert-vsetvl-phi-opt`
```suggestion
    "riscv-vsetvl-after-rvv-regalloc", cl::Hidden,
```

https://github.com/llvm/llvm-project/pull/70549