Re-land "[CodeGen][ExpandMemcmp][NFC] Allow memcmp to expand to vector loads (1)"

Compute the actual decomposition only after deciding whether to expand
of not. Else, it's easy to make the compiler OOM with:
`memcpy(dst, src, 0xffffffffffffffff);`, which typically happens if
someone mistakenly passes a negative value. Add a test.

This reverts commit f8fc02fb.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316567 91177308-0d34-0410-b5e6-96231b3b80d8