[InstCombine] peek through bitcasted vector/array pointer GEP operand

The bitcast may be interfering with other combines or vectorization 
as shown in PR16739:
https://bugs.llvm.org/show_bug.cgi?id=16739

Most pointer-related optimizations are probably able to look through 
this bitcast, but removing the bitcast shrinks the IR, so it's at
least a size savings.

Differential Revision: https://reviews.llvm.org/D44833

llvm-svn: 330237