MIR: Stop reinitializing target information for every use

Every time a physical register reference was parsed, this would
initialize a string map for every register in in target, and discard
it for the next. The same applies for the other fields initialized
from target information.

Follow along with how the function state is tracked, and add a new
tracking class for target information.

The string->register class/register bank for some reason were kept
separately, so track them in the same place.

llvm-svn: 355970