[MemCpyOpt] Promote both memset/memcpy sizes if differently typed.

Followup to r235232, which caused PR23278.

We can't assume the memset and memcpy sizes have the same type, as
nothing in the language reference prevents that.
Instead, zext both to i64 if they disagree.

While there, robustify tests by using i8 %c rather than i8 0 for the
memset character.

llvm-svn: 235258