A number of improvements in mips64 load immediate macro is added per
suggestions from MIPS ART team. Also fix Subu and Dsubu macro, add a
test for Subu and Dsubu and make minor code adjustments.

BUG=
TEST=cctest/test-assembler-mips/li_macro
     cctest/test-assembler-mips/Subu
     cctest/test-assembler-mips/Dsubu

Review-Url: https://codereview.chromium.org/2892163002
Cr-Commit-Position: refs/heads/master@{#45493}