[GlobalOpt] Fix the load types when OptimizeGlobalAddressOfMalloc

Currently, in OptimizeGlobalAddressOfMalloc, the transformation for global loads assumes that they have the same Type. With the support of ConstantExpr (https://reviews.llvm.org/D106589), this may not be true any more (as seen in the test case), and we miss the code to handle this, This is to fix that.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D107397