[mips] Optimize stack pointer adjustments.
Instead of always using addu to adjust the stack pointer when the
size out is of the range of an addiu instruction, use subu so that
a smaller constant can be generated.
This can give savings of ~3 instructions whenever a function has a
a stack frame whose size is out of range of an addiu instruction.
This change may break some naive stack unwinders.
Partially resolves PR/26291.
Thanks to David Chisnall for reporting the issue.
Reviewers: dsanders, vkalintiris
Differential Review: http://reviews.llvm.org/D21321