[LoongArch] Remove unnecessary increment operations

`HighMask` is the value that sets bits from `Msb+1` to 63 to 1, while
the other bits are set to 0.

Author: wangleiat

llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchMatInt.cpp

@@ -94,7 +94,7 @@ LoongArchMatInt::InstSeq LoongArchMatInt::generateInstSeq(int64_t Val) {
 
   uint64_t Msb = 32;
   uint64_t HighMask = ~((1ULL << (Msb + 1)) - 1);
-  for (; Msb < 64; ++Msb, HighMask = (HighMask << 1) + 1) {
+  for (; Msb < 64; ++Msb, HighMask = HighMask << 1) {
     for (uint64_t Lsb = Msb; Lsb > 0; --Lsb) {
       uint64_t LowMask = (1ULL << Lsb) - 1;
       uint64_t Mask = HighMask | LowMask;