[RISCV][SDAG] Fix constant narrowing when narrowing loads (#69015)

When narrowing logic ops(OR/XOR) with constant rhs, `DAGCombiner` will fixup the constant rhs node.
It is incorrect when lhs is also a constant. For example, we will incorrectly replace `xor OpaqueConstant:i64<8191>, Constant:i64<-1>` with `xor (and OpaqueConstant:i64<8191>, Constant:i64<65535>), Constant:i64<-1>`.

Fixes #68855.