[RISCV] Remove some unnecessary casts from int64_t to uint64_t. NFC

We have a lot of casts near this to avoid undefined behavior or
arithmetic on arbitrary signed integers, but the casts removed here
don't appear to be necessary.

Author: topperc

llvm/lib/Target/RISCV/MCTargetDesc/RISCVMatInt.cpp

@@ -134,7 +134,7 @@ static void generateInstSeqImpl(int64_t Val, const MCSubtargetInfo &STI,
     }
 
     // Try to use SLLI_UW for Val when it is uint32 but not int32.
-    if (isUInt<32>((uint64_t)Val) && !isInt<32>((uint64_t)Val) &&
+    if (isUInt<32>(Val) && !isInt<32>(Val) &&
         STI.hasFeature(RISCV::FeatureStdExtZba)) {
       // Use LUI+ADDI or LUI to compose, then clear the upper 32 bits with
       // SLLI_UW.