[MLIR][LLVM] Add simple folders for bitcast/addrspacecast/gep

Add 5 simple folders
* bitcast(x : T0, T0) -> x
* addrcast(x : T0, T0) -> x
* bitcast(bitcast(x : T0, T1), T0) -> x
* addrcast(addrcast(x : T0, T1), T0) -> x
* gep %x:T, 0 -> %x:T

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D116715