[RISCV] Fix assertion failure when zcmp extension is enabled.

Before accessing "getOpcode" thorugh machine instruction, check if the iterator
has reached the end of Machine basic block otherwise we will crash at the
assertion `!NodePtr->isKnownSentinel()`.

The above assertion is hit in "Prologue/Epilogue Insertion & Frame Finalization
pass".

Reviewed By: craig.topper, wangpc

Differential Revision: https://reviews.llvm.org/D158256