Replace hard-coded uses of `kScratchDoubleReg`, `kScratchDoubleReg2` and
`kScratchQuadReg` with the safer `UseScratchRegisterScope`. The reason for doing
this is to be able to safely use these scratch registers inside the assembler
without having to worry about the code generator using them too.

For instance, using this scope showed us that `TryInlineTruncateDoubleToI` is
using a FP scratch register while the caller, the `DoubleToI` stub, is using it
too. We are safe only because the stub passes the scratch register to
`TryInlineTruncateDoubleToI` as an input. Using the scope forces us to
explicitely use the input register instead of acquiring a new scratch.

Bug: v8:6553
Change-Id: I84c53cd851d31ea33b0e3ef398d7a858b7e3e3c4
Reviewed-on: https://chromium-review.googlesource.com/895460Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#51061}