[RISCV] Use the new chain when converting a fixed RVV load

When building the final merged node, we were using the original chain
rather than the output chain of the new operation. After some collapsing
of the chain this could cause the loads be incorrectly scheduled respect
to later stores.

This was uncovered by SingleSource/Regression/C/gcc-c-torture/execute/pr36038.c
of the llvm testsuite.

https://reviews.llvm.org/D125560