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.

llvm-svn: 175667