[llvm] [RISCV] Add 2^N + 2^M expanding pattern for mul (PR #137954)

[Iris Shi via llvm-commits]

================
@@ -494,6 +494,36 @@ define i32 @muli32_p14(i32 %a) nounwind {
   ret i32 %1
 }
 
+define i32 @muli32_p18(i32 %a) nounwind {
----------------
el-ev wrote:

The tests are added in the first commit in this pr and updated later, is that enough?

https://github.com/llvm/llvm-project/pull/137954