[llvm] [ARM64EC] Warn on using disallowed registers in assembly src. (PR #93618)

[Daniel Paoliello via llvm-commits]

================
@@ -5335,8 +5461,8 @@ bool AArch64AsmParser::validateInstruction(MCInst &Inst, SMLoc &IDLoc,
             (Reg >= AArch64::S16 && Reg <= AArch64::S31) ||
             (Reg >= AArch64::H16 && Reg <= AArch64::H31) ||
             (Reg >= AArch64::B16 && Reg <= AArch64::B31)) {
-          Warning(IDLoc, "this instruction uses disallowed registers.");
-          break;
+          Warning(IDLoc, "register " + getArm64ECDisallowedRegName(Reg) +
----------------
dpaoliello wrote:

```suggestion
          Warning(IDLoc, "register " + RI->getName(Reg) +
```

https://github.com/llvm/llvm-project/pull/93618