Add UseScratchRegisterScope for MIPS and use it instead of using at register directly.

Original commit message:
`Introduce a stripped down version of UseScratchRegisterScope for ARM and use it
inside the assembler and macro-assembler. At the exception of the Call
instructions, we now use this scope instead of using the ip register
directly. This is inspired from how the ARM64 backend works.

In general, the benefit of doing this is we can catch cases where ip is being
used both by the caller and by the assembler. But more specifically, TurboFan
reserves r9 as an extra scratch register because ip can already be used by the
assembler. With this utility, we can isolate the cases in the code generator
which need an extra register and potentially fix them, allowing us to give r9
back to the register allocator.

This patch uncovered places in the assembler where we were using ip
unconditionally when we could have re-used the destination register instead.`

Bug: 
Change-Id: I1a35c1661579882801605337abfc95f75b47f052
Reviewed-on: https://chromium-review.googlesource.com/574923
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Cr-Commit-Position: refs/heads/master@{#46963}