[mips] Do not allow -opt-bisect-limit to skip the PIC call optimization pass.
r282428 added the MipsOptimizePICCall as an opt-in pass that can be
skipped when using the -opt-bisect-limit option. However, this pass is
needed because it generates code that conforms to the o32 ABI
specification by using the $t9 register for PIC calls with JALR
This bug was exposed by the fact that skipFunction() also checks for
the "optnone" attribute. This caused functions with that attribute to
break the requirements of the o32 ABI.