[RISCV] Don't getDebugLoc for the end node of MBB iterator

Because of shrink wrapping, the block to insert epilog may don't have
instructions (Only debug instructions). And the position to insert may
point to MBB.end() that don't have a DebugLoc. This patch fix this
problem.

The test program was copied from the issue:https://github.com/llvm/llvm-project/issues/53662

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D123679