Renames IsDouble* predicates to IsFP*.
Adds specific IsFloat*, IsDouble*, and IsSimd128* predicates.
Adds specific GetFloatRegister, GetDoubleRegister, and
GetSimd128Register methods.

This is mostly a mechanical renaming of IsDouble* to IsFP* methods.

This shouldn't change code generation at all. All fp registers are still
treated as double registers.

LOG=N
BUG=v8:4124

Review-Url: https://codereview.chromium.org/1959763002
Cr-Commit-Position: refs/heads/master@{#36146}