[llvm] Replace vmv.s.x DAG combines with patterns. NFC (PR #71503)

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

The change looks good. But I have no preference to both.

For MacherTable, my opinion is that we should avoid using TableGen patterns for RVV optimizations since the pattern numbers are usually multipled by **7** (which is the number of register groups), and sometimes **7 * 4 = 28** when the instructions are related to `SEW`. Only basic patterns (those for instrinsics, `*_VL`s and standard `SDNode`s) should be put in TableGen.
cc @topperc 

https://github.com/llvm/llvm-project/pull/71503