[llvm] [RISCV][CG]Use processShuffleMasks for per-register shuffles (PR #121765)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> > Rework of the reverted #120803
> 
> Can you summarize the functional fix? What changed? _In particular_, which is the test which catches the prior functional bug?

Incorrectly calculated the destination registers. To fix this had to introduce a new boolean flag, that shows the start of filling a new destination register.

https://github.com/llvm/llvm-project/pull/121765