[RISCV] Fix miscompile in SExtWRemoval due to early return ignoring other sources

This code is walking back through a worklist of sources. All of the sources need to be sign extending for the result to be true. We had a case which returned rather than continued, which causes a miscompile when another source was not sign extended. The flawed logic was introduced in Dec 22, by change 844430bc.

This was recently exposed in a stage2 build of llvm-tablegen when we switched from using llvm::Optional to std::optional. The stars aligned in just the wrong way, and we started actively miscompiling idiomatic optional usage. std::optional<uint32_t> appears to use the top 32 bits of the word on RV64 for its tag.

Differential Revision: https://reviews.llvm.org/D143594