[RISCV] Fix crash in PEI with empty entry block with Zcmp (#72117)

We check the opcode of the first instruction in the block where the
prologue is inserted without checking if the iterator points to any
instructions. When the basic block is empty, that causes a crash. One
way the prologue block can be empty is when it starts with a call to
__builtin_readcyclecounter on RV32 since that produces a loop.

Co-authored-by: Nemanja Ivanovic <nemanja@synopsys.com>