[llvm] RISCV: Replace most Specifier constants with relocation types (PR #138644)

[Sam Elliott via llvm-commits]

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@@ -15,34 +15,21 @@
 #define LLVM_LIB_TARGET_RISCV_MCTARGETDESC_RISCVMCEXPR_H
 
 #include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCFixup.h"
 
 namespace llvm {
 
 class StringRef;
 
 class RISCVMCExpr : public MCTargetExpr {
 public:
-  enum Specifier : uint8_t {
+  using Specifier = uint16_t;
+  enum {
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lenary wrote:

Please can we have a comment for when you need a `VK` and when you can just use the relocation code directly. One of the cases is vendor relocations (which are not identified by just their relocation id), but I don't understand e.g. what makes the `VK_*_LO` variants special, or the `VK_CALL`. 

https://github.com/llvm/llvm-project/pull/138644