Improve Mips JIT.

Implement encoder methods getJumpTargetOpValue and getBranchTargetOpValue
for jmptarget and brtarget Mips tablegen operand types in the code emitter
for old-style JIT. Rename the pc relative relocation for branches - new
name is Mips::reloc_mips_pc16.

Patch by Sasa Stankovic

llvm-svn: 147382