[fast-isel] Remove an invalid assert.

If the memcpy has an odd length with an alignment of 2, this would incorrectly
assert on the last 1 byte copy.
rdar://13202135

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175459 91177308-0d34-0410-b5e6-96231b3b80d8