[SimplifyCFG] put the optional assumption cache pointer in the options struct; NFCI

[SimplifyCFG] put the optional assumption cache pointer in the options struct; NFCI

This is a follow-up to https://reviews.llvm.org/D38138.

I fixed the capitalization of some functions because we're changing those
lines anyway and that helped verify that we weren't accidentally dropping
any options by using default param values.

llvm-svn: 314930