[llvm-exegesis][NFC] Some code style cleanup

Apply review comments of https://reviews.llvm.org/D54185 to other target as well, specifically:

1. make anonymous namespaces as small as possible, avoid using static inside anonymous namespaces
2. Add missing header to some files
3. GetLoadImmediateOpcodem-> getLoadImmediateOpcode
4. Fix typo

Differential Revision: https://reviews.llvm.org/D54343

llvm-svn: 347309