Since `at` register is used a lot in macro-assembler-mips, change
usage of this register with `kScratchReg` and `kScratchReg2`.
Also, remove TODO comments for unaligned memory access, since there
is nothing that can be done about it.

Change-Id: Ibf55c04a1f53521f34dfb483294de3010a0120c6
Reviewed-on: https://chromium-review.googlesource.com/1059347Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#53181}