[PatternMatch] Use dyn_cast in CastInst_match instead of checking opcode. NFC (#79878)

Using dyn_cast allows us to use CastInst::getOperand instead of
Instruction::getOperand. This is more efficient since
CastInst::getOperand doesn't need to check how the operands are stored.
Instruction::getOperand has to consider HungOffUses.