[mips][mips64r6] Use JALR for indirect branches instead of JR (which is not available on MIPS32r6/MIPS64r6)

Summary:
This completes the change to use JALR instead of JR on MIPS32r6/MIPS64r6.

Reviewers: jkolek, vmedic, zoran.jovanovic, dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4269