MachineInstr: Print name for subreg index in SUBREG_TO_REG
MachineInstr: Print name for subreg index in SUBREG_TO_REG

SUBREG_TO_REG takes a subregister index as 3rd operand, print the name
instead of a number. We already do the same for INSERT_SUBREG and
REG_SEQUENCE.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@291481 91177308-0d34-0410-b5e6-96231b3b80d8

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