Fix PR4748: don't fold gep(bitcast(x)) into bitcast(gep) when x

is itself a bitcast.  Since we have gep(bitcast(bitcast(y))) in this
case, just wait for the two bitcasts to get zapped.  This prevents
instcombine from confusing some aliasing stuff, and allows it to
directly eliminate the load in the testcase.

llvm-svn: 80508