[RISCV] Use MnemonicAlias for 'fssr' and 'frsr'. NFC (#131877)

Author: topperc

llvm/lib/Target/RISCV/RISCVInstrInfoF.td

@@ -425,11 +425,9 @@ def : InstAlias<"frcsr $rd",      (CSRRS GPR:$rd, SysRegFCSR.Encoding, X0), 2>;
 def : InstAlias<"fscsr $rd, $rs", (CSRRW GPR:$rd, SysRegFCSR.Encoding, GPR:$rs)>;
 def : InstAlias<"fscsr $rs",      (CSRRW      X0, SysRegFCSR.Encoding, GPR:$rs), 2>;
 
-// frsr, fssr are obsolete aliases replaced by frcsr, fscsr, so give them
-// zero weight.
-def : InstAlias<"frsr $rd",       (CSRRS GPR:$rd, SysRegFCSR.Encoding, X0), 0>;
-def : InstAlias<"fssr $rd, $rs",  (CSRRW GPR:$rd, SysRegFCSR.Encoding, GPR:$rs), 0>;
-def : InstAlias<"fssr $rs",       (CSRRW      X0, SysRegFCSR.Encoding, GPR:$rs), 0>;
+// frsr, fssr are obsolete aliases replaced by frcsr, fscsr.
+def : MnemonicAlias<"frsr", "frcsr">;
+def : MnemonicAlias<"fssr", "fscsr">;
 
 def : InstAlias<"frrm $rd",        (CSRRS  GPR:$rd, SysRegFRM.Encoding, X0), 2>;
 def : InstAlias<"fsrm $rd, $rs",   (CSRRW  GPR:$rd, SysRegFRM.Encoding, GPR:$rs)>;