Pointer types were treated as zero-size by MergeICmps

Summary:
The visitICmp analysis function would record compares of pointer types, as size 0. This causes the resulting memcmp() call to have the wrong total size.
Found with "self-build" of clang/LLVM on Windows.

Reviewers: christylee, trentxintong, courbet

Reviewed By: courbet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53536

llvm-svn: 345413