[AArch64] SVEIntrinsicOpts - use range loop and cast<> instead of dyn_cast<>...

[AArch64] SVEIntrinsicOpts - use range loop and cast<> instead of dyn_cast<> for dereferenced pointer. NFCI.

Don't directly dereference a dyn_cast<> - use cast<> so we assert for the correct type.

Also, simplify the for loop to a range loop.

Fixes clang static analyzer warning.