Shrink number of instruction in ByteSwap macro for some cases.
Allow that input and output registers can be the same.
Extend test to cover all test cases.

Change-Id: I7e0b86988fb73eed604751ffd89657cdff4abc3c
Reviewed-on: https://chromium-review.googlesource.com/951726Reviewed-by: Sreten Kovacevic <sreten.kovacevic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#51787}