[MIRPrinter] Use %subreg.xxx syntax for subregister index operands

Summary:
Print %subreg.<subregidxname> instead of just the subregister
index when printing immediate operands corresponding to subreg
indices in INSERT_SUBREG, EXTRACT_SUBREG, SUBREG_TO_REG and
REG_SEQUENCE.

Reviewers: qcolombet, MatzeB

Reviewed By: MatzeB

Subscribers: nhaehnle, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D39696

llvm-svn: 317513