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

[Craig Topper via llvm-commits]

topperc wrote:

Can the new signature of `lookupSysRegByEncoding` return the std::pair from `equal_range` instead of needing to copy into a vector?

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