[llvm] [clang] [SPARC] Support reserving arbitrary general purpose registers (PR #74927)

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@@ -29,6 +29,12 @@ namespace llvm {
 class StringRef;
 
 class SparcSubtarget : public SparcGenSubtargetInfo {
+  // Reserve*Register[i] - *#i is not available as a general purpose register.
+  BitVector ReserveGRegister;
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koachan wrote:

It should be possible, however my concern is that [the flags](https://github.com/llvm/llvm-project/pull/74927/files#diff-cf43b5b875187dc9a8dc30d7cd08b1595d63286ef469df03d7f89c92fa294fb5R68-R81) need to be defined in terms of register names, and tablegen seems to not like doing arithmetic inside the `"Reserve*Register["#i#"]"` parts?

https://github.com/llvm/llvm-project/pull/74927