Optimizing: Untangle `RegisterSet` from `Location`.

Remove `RegisterSet` members that take a `Location` and add
replacement `Add{Core,Fpu}Register()` member functions. The
`Remove()` was dead code, so we don't need any replacement.

Rename `Add{Core,Fpu}Registers` to stress the `Set` and
avoid a single-character difference with the new functions.

Test: testrunner.py -b --host-gtest --host --optimizing
Flag: EXEMPT PURE_REFACTOR
Change-Id: I37c3e60419ac6e0693342cae92c6bc122a1a2b15