Correctly save and restore FP registers in cctest/ConvertDToI to
avoid accidental register overwriting by the generated code.

Note: This failure is manifested only in R6 mode.

TEST=test/cctest/interpreter/test-interpreter
BUG=

Review URL: https://codereview.chromium.org/1337023002

Cr-Commit-Position: refs/heads/master@{#30700}