Extend memcpy expansion in Transform/Utils to handle wider operand types.

Adds loop expansions for known-size and unknown-sized memcpy calls, allowing the
target to provide the operand types through TTI callbacks. The default values
for the TTI callbacks use int8 operand types and matches the existing behaviour
if they aren't overridden by the target.

Differential revision: https://reviews.llvm.org/D32536

llvm-svn: 307346