[llvm] [Mips] Fix $gp was restored when used as global register variable (PR #201778)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,23 @@
+; RUN: llc -mtriple=mips64el -mcpu=mips64r2 < %s | FileCheck %s -check-prefix=MIPS64
+
+; Test that when a user declares $28 as a global register, assignments to it are
+; not optimizes away.
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arsenm wrote:

```suggestion
; not optimized away.
```

https://github.com/llvm/llvm-project/pull/201778