[llvm] [RISCV] Remove unnecessary dependencies on vtype register. (PR #146971)

[Mikhail Gudim via llvm-commits]

mgudim wrote:

@preames @topperc @lukel97 The comment at the top of RISCVInsertVSETVLI.cpp:

```
// Phase 2 uses the information from phase 1 to do a data flow analysis to
// propagate the VL/VTYPE changes through the function. This gives us the
// VL/VTYPE at the start of each basic block.
```

Could we just use reaching definitions analysis for that?

https://github.com/llvm/llvm-project/pull/146971