Changes some platform specific class names and uses typedefs
to define FloatRegister, DoubleRegister, etc.

This will be needed to do register allocation on ARM/MIPS where
registers combine to form larger registers.

LOG=N
BUG=v8:4124

Review-Url: https://codereview.chromium.org/1954953002
Cr-Commit-Position: refs/heads/master@{#36117}