[mips] Emit a JALR with $rd equal to $zero, instead of a JR in MIPS32R6.

Summary:
JR is an alias of JALR with $rd=0 in the R6 ISA. Also, this fixes recursive
builds in MIPS32R6.

Reviewers: dsanders, sdardis

Subscribers: jfb, dschuff, dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D21370

llvm-svn: 273085