Floating point param and return registers should be within the
compiler's allocatable set.

TEST=cctest/test-run-wasm-js/Run_Float64Add_jswrapped
R=titzer@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1714223002

Cr-Commit-Position: refs/heads/master@{#34194}