[llvm] [RISCV] Don't custom lower direct fp_extends where possible (PR #125644)

[Luke Lau via llvm-commits]

lukel97 wrote:

I've reworked this to convert the fp_extends to _vl nodes in PreprocessISelDAG so we don't need to add any more patterns. And we can remove the VPatWidenReductionVL_RM patterns too now that we don't need to match fp_extend.

https://github.com/llvm/llvm-project/pull/125644