arch-arm: Fix decoding of LDFF1x scalar plus scalar

First-faulting loads do allow Rm == 0x1f.

Change-Id: Ib9bcb55e126653813fdbb7c29970af23a2471ebb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23803
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/isa/formats/sve_2nd_level.isa

@@ -3132,10 +3132,6 @@ namespace Aarch64
         IntRegIndex rm = (IntRegIndex) (uint8_t) bits(machInst, 20, 16);
         IntRegIndex pg = (IntRegIndex) (uint8_t) bits(machInst, 12, 10);
 
-        if (rm == 0x1f) {
-            return new Unknown64(machInst);
-        }
-
         return decodeSveContigLoadSSInsts<SveContigFFLoadSS>(
             bits(machInst, 24, 21), machInst, zt, pg, rn, rm, true);
     }  // decodeSveContigFFLoadSS