[RISCV] Use uint64_t for Insn in getInstruction32 and getInstruction16. NFC (#146619)

Insn is passed to decodeInstruction which is a template function based
on the type of Insn. By using uint64_t we ensure only one version of
decodeInstruction is created. This reduces the file size of
RISCVDisassembler.cpp.o by ~25% in my local build.

Author: topperc

llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp

@@ -811,7 +811,9 @@ DecodeStatus RISCVDisassembler::getInstruction32(MCInst &MI, uint64_t &Size,
   }
   Size = 4;
 
-  uint32_t Insn = support::endian::read32le(Bytes.data());
+  // Use uint64_t to match getInstruction48. decodeInstruction is templated
+  // on the Insn type.
+  uint64_t Insn = support::endian::read32le(Bytes.data());
 
   for (const DecoderListEntry &Entry : DecoderList32) {
     if (!Entry.haveContainedFeatures(STI.getFeatureBits()))
@@ -857,7 +859,9 @@ DecodeStatus RISCVDisassembler::getInstruction16(MCInst &MI, uint64_t &Size,
   }
   Size = 2;
 
-  uint32_t Insn = support::endian::read16le(Bytes.data());
+  // Use uint64_t to match getInstruction48. decodeInstruction is templated
+  // on the Insn type.
+  uint64_t Insn = support::endian::read16le(Bytes.data());
 
   for (const DecoderListEntry &Entry : DecoderList16) {
     if (!Entry.haveContainedFeatures(STI.getFeatureBits()))