[llvm] [Mips] Support "$sp" named register (PR #136821)

[via llvm-commits]

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@@ -22,8 +22,19 @@ define i32 @get_sp() {
   ret i32 %1
 }
 
+define i32 @get_$sp() {
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yingopq wrote:

I have appended test with other reg, can you help review again?

https://github.com/llvm/llvm-project/pull/136821