[llvm] [RISCV] Implement foward inserting save/restore FRM instructions. (PR #77744)

[Craig Topper via llvm-commits]

topperc wrote:

> > The advanced version of this is to perform cross basic block analysis for the starting rounding mode of each basic block.
> 
> I wonder if there's a way we could abstract the cross block analysis in RISCVInsertVSETVLI?

Or the one in VXRM insertion. I think the hard thing with FRM is managing the virtual register needed to hold the previous dynamic value.

https://github.com/llvm/llvm-project/pull/77744