[llvm] [MC] Make MCRegisterInfo::getLLVMRegNum return std::optional<MCRegister>. NFC (PR #107776)

[Aiden Grossman via llvm-commits]

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@@ -422,7 +422,7 @@ class MCRegisterInfo {
 
   /// Map a dwarf register back to a target register. Returns std::nullopt is
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boomanaiden154 wrote:

While you're at it, maybe `s/is/if` at the end of the line?

https://github.com/llvm/llvm-project/pull/107776