[llvm] [RISCV]Add support for resolving encoding conflicts among vendor specific CSRs (PR #96174)

[Craig Topper via llvm-commits]

topperc wrote:

Can `lookupSysRegByEncoding` use `equal_range` instead of calling `lower_bound` and `upper_bound`? Then iterate the returned range to push into the vector?

https://github.com/llvm/llvm-project/pull/96174