Replace {G}CSRXCHG with [G]CSRXCHG to align with LoongArch ISA Manual style

Author: heiher

llvm/lib/Target/LoongArch/LoongArchRegisterInfo.td

@@ -127,9 +127,9 @@ def GPRT : GPRRegisterClass<(add // a0...a7, t0...t8
 // prediction.
 def GPRJR : GPRRegisterClass<(sub GPR, R1)>;
 
-// Don't use R0 or R1 for the rj operand of {G}CSRXCHG, because when rj is
-// encoded as 0 or 1, the instruction is interpreted as {G}CSRRD or {G}CSRWR,
-// respectively, rather than {G}CSRXCHG.
+// Don't use R0 or R1 for the rj operand of [G]CSRXCHG, because when rj is
+// encoded as 0 or 1, the instruction is interpreted as [G]CSRRD or [G]CSRWR,
+// respectively, rather than [G]CSRXCHG.
 def GPRNoR0R1 : GPRRegisterClass<(sub GPR, R0, R1)>;
 
 // Floating point registers