[ExpandMemcmp] rename variables and add hook to override pref for number of loads per block; NFC

The preference only applies to 'memcmp() == 0' expansion, so try to make that clearer.
x86 will likely benefit by increasing the default value from '1' to '2' as seen in PR33325:
https://bugs.llvm.org/show_bug.cgi?id=33325
...so that is the planned follow-up to this clean-up step.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321756 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed