[MIPS][LLVM-MC] Fix JR encoding for MIPSR6 ISA

Summary: The assembler was picking the wrong JR variant because the pre-R6 one was still enabled at R6.

Author: nitesh.jain
Reviewers: vkalintiris, dsanders
Subscribers: dsanders, llvm-commits, mohit.bhakkad, sagar, bhushan, jaydeep
Differential: D18387
llvm-svn: 265134