[mips] Relax jalr/jr instructions using R_MIPS_JALR relocation

The R_MIPS_JALR relocation denotes jalr/jr instructions in position
independent code. Both these instructions take a target's address from
the $25 register. If offset to the target symbol fits into the 18-bits,
it's more efficient to replace jalr/jr by bal/b instructions.

Differential Revision: https://reviews.llvm.org/D68057

llvm-svn: 372951