[mips] For PIC code convert unconditional jump to unconditional branch

Unconditional branch uses relative addressing which is the right choice
in case of position independent code.

This is a fix for the bug:
https://dmz-portal.mips.com/bugz/show_bug.cgi?id=2445

Differential revision: https://reviews.llvm.org/D27483

llvm-svn: 289448