An optimization in li checks if a value has all their "n" LSBs set to
one, and their "64-n" MSBs set to zero. This is done by checking
base::bits::IsPowerOfTwo(value + 1), but this doesn't cover the case
when only MSB is zero, and other bits are one. Added another check that
covers the missing case, this also fixes Dsubu test.

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

Change-Id: I97b8fb4984af3b7f974ba901e3a1a9c89e925805
Reviewed-on: https://chromium-review.googlesource.com/598228Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Commit-Queue: Miran Karić <Miran.Karic@imgtec.com>
Cr-Commit-Position: refs/heads/master@{#47134}