Explain partial byte extraction logic.

This is a follow-up to #92506.

Author: jreiffers

llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp

@@ -1851,6 +1851,9 @@ void NVPTXAsmPrinter::bufferLEByte(const Constant *CPV, int Bytes,
   auto AddIntToBuffer = [AggBuffer, Bytes](const APInt &Val) {
     size_t NumBytes = (Val.getBitWidth() + 7) / 8;
     SmallVector<unsigned char, 16> Buf(NumBytes);
+    // `extractBitsAsZExtValue` does not allow the extraction of bits beyond the
+    // input's bit width. We handle the last byte separately, so we never
+    // request out of bounds bits.
     for (unsigned I = 0; I < NumBytes - 1; ++I) {
       Buf[I] = Val.extractBitsAsZExtValue(8, I * 8);
     }