The MIPS IAS can now pass 'ninja check-all' and recurse now that the immediates
are all range checked properly.
Unfortunately we can't enable it by default for 64-bit targets yet since the N32
ABI is still very buggy and this also means we can't enable it for N64 either
because we can't distinguish between N32 and N64 in the relevant code.
Depends on D18753