Update TargetLowering ivars for name policy.

Update TargetLowering ivars for name policy.

http://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly

ivars should be camel-case and start with an upper-case letter. A few in
TargetLowering were starting with a lower-case letter.

No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175667 91177308-0d34-0410-b5e6-96231b3b80d8