For int16 imm values Subu would emit addiu with -imm value, but doing
this with min_int16 would overflow and produce incorrect result. This is
fixed by checking if -imm is int16. A test for this case is created.

An optimization is also added for values imm where we cannot just emit
addiu and loading -imm to a register takes one instruction using ori.
Then instead of loading imm with lui;ori and subtracting with subu, we
can load -imm with ori and add with addu.

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

Review-Url: https://codereview.chromium.org/2845043002
Cr-Commit-Position: refs/heads/master@{#44932}